t-247: Add Provider abstraction for multi-backend LLM support

- Create Omni/Agent/Provider.hs with unified Provider interface
- Support OpenRouter (cloud), Ollama (local), Amp (subprocess stub)
- Add runAgentWithProvider to Engine.hs for Provider-based execution
- Add EngineType to Core.hs (EngineOpenRouter, EngineOllama, EngineAmp)
- Add --engine flag to 'jr work' command
- Worker.hs dispatches to appropriate provider based on engine type

Usage:
  jr work <task-id>                    # OpenRouter (default)
  jr work <task-id> --engine=ollama    # Local Ollama
  jr work <task-id> --engine=amp       # Amp CLI (stub)

1 files changed, 386 insertions, 0 deletions

diff --git a/Omni/Agent/Provider.hs b/Omni/Agent/Provider.hs
new file mode 100644
index 0000000..a8a5381
--- /dev/null
+++ b/Omni/Agent/Provider.hs
@@ -0,0 +1,386 @@
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE NoImplicitPrelude #-}
+
+-- | LLM Provider abstraction for multi-backend support.
+--
+-- Supports multiple LLM backends:
+-- - OpenRouter (cloud API, multiple models)
+-- - Ollama (local models)
+-- - Amp CLI (subprocess)
+--
+-- : out omni-agent-provider
+-- : dep aeson
+-- : dep http-conduit
+-- : dep case-insensitive
+module Omni.Agent.Provider
+  ( Provider (..),
+    ProviderConfig (..),
+    ChatResult (..),
+    Message (..),
+    Role (..),
+    ToolCall (..),
+    FunctionCall (..),
+    Usage (..),
+    ToolApi (..),
+    defaultOpenRouter,
+    defaultOllama,
+    chat,
+    chatWithUsage,
+    main,
+    test,
+  )
+where
+
+import Alpha
+import Data.Aeson ((.=))
+import qualified Data.Aeson as Aeson
+import qualified Data.Aeson.KeyMap as KeyMap
+import qualified Data.ByteString.Lazy as BL
+import qualified Data.CaseInsensitive as CI
+import qualified Data.Text as Text
+import qualified Data.Text.Encoding as TE
+import qualified Network.HTTP.Simple as HTTP
+import qualified Omni.Test as Test
+
+main :: IO ()
+main = Test.run test
+
+test :: Test.Tree
+test =
+  Test.group
+    "Omni.Agent.Provider"
+    [ Test.unit "defaultOpenRouter has correct endpoint" <| do
+        case defaultOpenRouter "" "test-model" of
+          OpenRouter cfg -> providerBaseUrl cfg Test.@=? "https://openrouter.ai/api/v1"
+          _ -> Test.assertFailure "Expected OpenRouter",
+      Test.unit "defaultOllama has correct endpoint" <| do
+        case defaultOllama "test-model" of
+          Ollama cfg -> providerBaseUrl cfg Test.@=? "http://localhost:11434"
+          _ -> Test.assertFailure "Expected Ollama",
+      Test.unit "ChatResult preserves message" <| do
+        let msg = Message User "test" Nothing Nothing
+            result = ChatResult msg Nothing
+        chatMessage result Test.@=? msg
+    ]
+
+data Provider
+  = OpenRouter ProviderConfig
+  | Ollama ProviderConfig
+  | AmpCLI FilePath
+  deriving (Show, Eq, Generic)
+
+data ProviderConfig = ProviderConfig
+  { providerBaseUrl :: Text,
+    providerApiKey :: Text,
+    providerModel :: Text,
+    providerExtraHeaders :: [(ByteString, ByteString)]
+  }
+  deriving (Show, Eq, Generic)
+
+instance Aeson.ToJSON ProviderConfig where
+  toJSON c =
+    Aeson.object
+      [ "baseUrl" .= providerBaseUrl c,
+        "apiKey" .= providerApiKey c,
+        "model" .= providerModel c
+      ]
+
+instance Aeson.FromJSON ProviderConfig where
+  parseJSON =
+    Aeson.withObject "ProviderConfig" <| \v ->
+      (ProviderConfig </ (v Aeson..: "baseUrl"))
+        <*> (v Aeson..: "apiKey")
+        <*> (v Aeson..: "model")
+        <*> pure []
+
+defaultOpenRouter :: Text -> Text -> Provider
+defaultOpenRouter apiKey model =
+  OpenRouter
+    ProviderConfig
+      { providerBaseUrl = "https://openrouter.ai/api/v1",
+        providerApiKey = apiKey,
+        providerModel = model,
+        providerExtraHeaders =
+          [ ("HTTP-Referer", "https://omni.dev"),
+            ("X-Title", "Omni Agent")
+          ]
+      }
+
+defaultOllama :: Text -> Provider
+defaultOllama model =
+  Ollama
+    ProviderConfig
+      { providerBaseUrl = "http://localhost:11434",
+        providerApiKey = "",
+        providerModel = model,
+        providerExtraHeaders = []
+      }
+
+data Role = System | User | Assistant | ToolRole
+  deriving (Show, Eq, Generic)
+
+instance Aeson.ToJSON Role where
+  toJSON System = Aeson.String "system"
+  toJSON User = Aeson.String "user"
+  toJSON Assistant = Aeson.String "assistant"
+  toJSON ToolRole = Aeson.String "tool"
+
+instance Aeson.FromJSON Role where
+  parseJSON = Aeson.withText "Role" parseRole
+    where
+      parseRole "system" = pure System
+      parseRole "user" = pure User
+      parseRole "assistant" = pure Assistant
+      parseRole "tool" = pure ToolRole
+      parseRole _ = empty
+
+data Message = Message
+  { msgRole :: Role,
+    msgContent :: Text,
+    msgToolCalls :: Maybe [ToolCall],
+    msgToolCallId :: Maybe Text
+  }
+  deriving (Show, Eq, Generic)
+
+instance Aeson.ToJSON Message where
+  toJSON m =
+    Aeson.object
+      <| catMaybes
+        [ Just ("role" .= msgRole m),
+          Just ("content" .= msgContent m),
+          ("tool_calls" .=) </ msgToolCalls m,
+          ("tool_call_id" .=) </ msgToolCallId m
+        ]
+
+instance Aeson.FromJSON Message where
+  parseJSON =
+    Aeson.withObject "Message" <| \v ->
+      (Message </ (v Aeson..: "role"))
+        <*> (v Aeson..:? "content" Aeson..!= "")
+        <*> (v Aeson..:? "tool_calls")
+        <*> (v Aeson..:? "tool_call_id")
+
+data ToolCall = ToolCall
+  { tcId :: Text,
+    tcType :: Text,
+    tcFunction :: FunctionCall
+  }
+  deriving (Show, Eq, Generic)
+
+instance Aeson.ToJSON ToolCall where
+  toJSON tc =
+    Aeson.object
+      [ "id" .= tcId tc,
+        "type" .= tcType tc,
+        "function" .= tcFunction tc
+      ]
+
+instance Aeson.FromJSON ToolCall where
+  parseJSON =
+    Aeson.withObject "ToolCall" <| \v ->
+      (ToolCall </ (v Aeson..: "id"))
+        <*> (v Aeson..:? "type" Aeson..!= "function")
+        <*> (v Aeson..: "function")
+
+data FunctionCall = FunctionCall
+  { fcName :: Text,
+    fcArguments :: Text
+  }
+  deriving (Show, Eq, Generic)
+
+instance Aeson.ToJSON FunctionCall where
+  toJSON fc =
+    Aeson.object
+      [ "name" .= fcName fc,
+        "arguments" .= fcArguments fc
+      ]
+
+instance Aeson.FromJSON FunctionCall where
+  parseJSON =
+    Aeson.withObject "FunctionCall" <| \v ->
+      (FunctionCall </ (v Aeson..: "name"))
+        <*> (v Aeson..: "arguments")
+
+data Usage = Usage
+  { usagePromptTokens :: Int,
+    usageCompletionTokens :: Int,
+    usageTotalTokens :: Int,
+    usageCost :: Maybe Double
+  }
+  deriving (Show, Eq, Generic)
+
+instance Aeson.FromJSON Usage where
+  parseJSON =
+    Aeson.withObject "Usage" <| \v ->
+      (Usage </ (v Aeson..: "prompt_tokens"))
+        <*> (v Aeson..: "completion_tokens")
+        <*> (v Aeson..: "total_tokens")
+        <*> (v Aeson..:? "cost")
+
+data ChatResult = ChatResult
+  { chatMessage :: Message,
+    chatUsage :: Maybe Usage
+  }
+  deriving (Show, Eq)
+
+data ToolApi = ToolApi
+  { toolApiName :: Text,
+    toolApiDescription :: Text,
+    toolApiParameters :: Aeson.Value
+  }
+  deriving (Generic)
+
+instance Aeson.ToJSON ToolApi where
+  toJSON t =
+    Aeson.object
+      [ "type" .= ("function" :: Text),
+        "function"
+          .= Aeson.object
+            [ "name" .= toolApiName t,
+              "description" .= toolApiDescription t,
+              "parameters" .= toolApiParameters t
+            ]
+      ]
+
+data ChatCompletionRequest = ChatCompletionRequest
+  { reqModel :: Text,
+    reqMessages :: [Message],
+    reqTools :: Maybe [ToolApi]
+  }
+  deriving (Generic)
+
+instance Aeson.ToJSON ChatCompletionRequest where
+  toJSON r =
+    Aeson.object
+      <| catMaybes
+        [ Just ("model" .= reqModel r),
+          Just ("messages" .= reqMessages r),
+          ("tools" .=) </ reqTools r,
+          Just ("usage" .= Aeson.object ["include" .= True])
+        ]
+
+data Choice = Choice
+  { choiceIndex :: Int,
+    choiceMessage :: Message,
+    choiceFinishReason :: Maybe Text
+  }
+  deriving (Show, Eq, Generic)
+
+instance Aeson.FromJSON Choice where
+  parseJSON =
+    Aeson.withObject "Choice" <| \v ->
+      (Choice </ (v Aeson..: "index"))
+        <*> (v Aeson..: "message")
+        <*> (v Aeson..:? "finish_reason")
+
+data ChatCompletionResponse = ChatCompletionResponse
+  { respId :: Text,
+    respChoices :: [Choice],
+    respModel :: Text,
+    respUsage :: Maybe Usage
+  }
+  deriving (Show, Eq, Generic)
+
+instance Aeson.FromJSON ChatCompletionResponse where
+  parseJSON =
+    Aeson.withObject "ChatCompletionResponse" <| \v ->
+      (ChatCompletionResponse </ (v Aeson..: "id"))
+        <*> (v Aeson..: "choices")
+        <*> (v Aeson..: "model")
+        <*> (v Aeson..:? "usage")
+
+chat :: Provider -> [ToolApi] -> [Message] -> IO (Either Text Message)
+chat provider tools messages = do
+  result <- chatWithUsage provider tools messages
+  pure (chatMessage </ result)
+
+chatWithUsage :: Provider -> [ToolApi] -> [Message] -> IO (Either Text ChatResult)
+chatWithUsage (OpenRouter cfg) tools messages = chatOpenAI cfg tools messages
+chatWithUsage (Ollama cfg) tools messages = chatOllama cfg tools messages
+chatWithUsage (AmpCLI _promptFile) _tools _messages = do
+  pure (Left "Amp CLI provider not yet implemented")
+
+chatOpenAI :: ProviderConfig -> [ToolApi] -> [Message] -> IO (Either Text ChatResult)
+chatOpenAI cfg tools messages = do
+  let url = Text.unpack (providerBaseUrl cfg) <> "/chat/completions"
+  req0 <- HTTP.parseRequest url
+  let body =
+        ChatCompletionRequest
+          { reqModel = providerModel cfg,
+            reqMessages = messages,
+            reqTools = if null tools then Nothing else Just tools
+          }
+      baseReq =
+        HTTP.setRequestMethod "POST"
+          <| HTTP.setRequestHeader "Content-Type" ["application/json"]
+          <| HTTP.setRequestHeader "Authorization" ["Bearer " <> TE.encodeUtf8 (providerApiKey cfg)]
+          <| HTTP.setRequestBodyLBS (Aeson.encode body)
+          <| req0
+      req = foldr addHeader baseReq (providerExtraHeaders cfg)
+      addHeader (name, value) = HTTP.addRequestHeader (CI.mk name) value
+
+  response <- HTTP.httpLBS req
+  let status = HTTP.getResponseStatusCode response
+  if status >= 200 && status < 300
+    then case Aeson.decode (HTTP.getResponseBody response) of
+      Just resp ->
+        case respChoices resp of
+          (c : _) -> pure (Right (ChatResult (choiceMessage c) (respUsage resp)))
+          [] -> pure (Left "No choices in response")
+      Nothing -> pure (Left "Failed to parse response")
+    else pure (Left ("HTTP error: " <> tshow status <> " - " <> TE.decodeUtf8 (BL.toStrict (HTTP.getResponseBody response))))
+
+chatOllama :: ProviderConfig -> [ToolApi] -> [Message] -> IO (Either Text ChatResult)
+chatOllama cfg tools messages = do
+  let url = Text.unpack (providerBaseUrl cfg) <> "/api/chat"
+  req0 <- HTTP.parseRequest url
+  let body =
+        Aeson.object
+          [ "model" .= providerModel cfg,
+            "messages" .= messages,
+            "tools" .= if null tools then Aeson.Null else Aeson.toJSON tools,
+            "stream" .= False
+          ]
+      req =
+        HTTP.setRequestMethod "POST"
+          <| HTTP.setRequestHeader "Content-Type" ["application/json"]
+          <| HTTP.setRequestBodyLBS (Aeson.encode body)
+          <| req0
+
+  response <- HTTP.httpLBS req
+  let status = HTTP.getResponseStatusCode response
+  if status >= 200 && status < 300
+    then case Aeson.decode (HTTP.getResponseBody response) of
+      Just resp -> parseOllamaResponse resp
+      Nothing -> pure (Left ("Failed to parse Ollama response: " <> TE.decodeUtf8 (BL.toStrict (HTTP.getResponseBody response))))
+    else pure (Left ("HTTP error: " <> tshow status <> " - " <> TE.decodeUtf8 (BL.toStrict (HTTP.getResponseBody response))))
+
+parseOllamaResponse :: Aeson.Value -> IO (Either Text ChatResult)
+parseOllamaResponse val =
+  case val of
+    Aeson.Object obj -> do
+      let msgResult = do
+            msgObj <- case KeyMap.lookup "message" obj of
+              Just m -> Right m
+              Nothing -> Left "No message in response"
+            case Aeson.fromJSON msgObj of
+              Aeson.Success msg -> Right msg
+              Aeson.Error e -> Left (Text.pack e)
+          usageResult = case KeyMap.lookup "prompt_eval_count" obj of
+            Just (Aeson.Number promptTokens) ->
+              case KeyMap.lookup "eval_count" obj of
+                Just (Aeson.Number evalTokens) ->
+                  Just
+                    Usage
+                      { usagePromptTokens = round promptTokens,
+                        usageCompletionTokens = round evalTokens,
+                        usageTotalTokens = round promptTokens + round evalTokens,
+                        usageCost = Nothing
+                      }
+                _ -> Nothing
+            _ -> Nothing
+      case msgResult of
+        Right msg -> pure (Right (ChatResult msg usageResult))
+        Left e -> pure (Left e)
+    _ -> pure (Left "Expected object response from Ollama")