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, 176 insertions, 0 deletions

diff --git a/Omni/Agent/Engine.hs b/Omni/Agent/Engine.hs
index 4ee5e5d..fe3b3d5 100644
--- a/Omni/Agent/Engine.hs
+++ b/Omni/Agent/Engine.hs
@@ -30,12 +30,15 @@ module Omni.Agent.Engine
     ChatCompletionResponse (..),
     Choice (..),
     Usage (..),
+    ToolApi (..),
+    encodeToolForApi,
     defaultLLM,
     defaultEngineConfig,
     defaultAgentConfig,
     defaultGuardrails,
     chat,
     runAgent,
+    runAgentWithProvider,
     main,
     test,
   )
@@ -51,6 +54,7 @@ import qualified Data.Map.Strict as Map
 import qualified Data.Text as Text
 import qualified Data.Text.Encoding as TE
 import qualified Network.HTTP.Simple as HTTP
+import qualified Omni.Agent.Provider as Provider
 import qualified Omni.Test as Test
 
 main :: IO ()
@@ -264,6 +268,14 @@ encodeToolForApi t =
       toolApiParameters = toolJsonSchema t
     }
 
+encodeToolForProvider :: Tool -> Provider.ToolApi
+encodeToolForProvider t =
+  Provider.ToolApi
+    { Provider.toolApiName = toolName t,
+      Provider.toolApiDescription = toolDescription t,
+      Provider.toolApiParameters = toolJsonSchema t
+    }
+
 data LLM = LLM
   { llmBaseUrl :: Text,
     llmApiKey :: Text,
@@ -809,3 +821,167 @@ estimateCost model tokens
   | "gpt-4" `Text.isInfixOf` model = fromIntegral tokens * 3 / 100000
   | "claude" `Text.isInfixOf` model = fromIntegral tokens * 3 / 100000
   | otherwise = fromIntegral tokens / 100000
+
+-- | Run agent with a Provider instead of LLM.
+-- This is the new preferred way to run agents with multiple backend support.
+runAgentWithProvider :: EngineConfig -> Provider.Provider -> AgentConfig -> Text -> IO (Either Text AgentResult)
+runAgentWithProvider engineCfg provider agentCfg userPrompt = do
+  let tools = agentTools agentCfg
+      toolApis = map encodeToolForProvider tools
+      toolMap = buildToolMap tools
+      systemMsg = providerMessage Provider.System (agentSystemPrompt agentCfg)
+      userMsg = providerMessage Provider.User userPrompt
+      initialMessages = [systemMsg, userMsg]
+
+  engineOnActivity engineCfg "Starting agent loop (Provider)"
+  loopProvider provider toolApis toolMap initialMessages 0 0 0 0.0 Map.empty 0 0
+  where
+    maxIter = agentMaxIterations agentCfg
+    guardrails' = agentGuardrails agentCfg
+
+    providerMessage :: Provider.Role -> Text -> Provider.Message
+    providerMessage role content = Provider.Message role content Nothing Nothing
+
+    loopProvider ::
+      Provider.Provider ->
+      [Provider.ToolApi] ->
+      Map.Map Text Tool ->
+      [Provider.Message] ->
+      Int ->
+      Int ->
+      Int ->
+      Double ->
+      Map.Map Text Int ->
+      Int ->
+      Int ->
+      IO (Either Text AgentResult)
+    loopProvider prov toolApis' toolMap msgs iteration totalCalls totalTokens totalCost toolCallCounts testFailures editFailures
+      | iteration >= maxIter = do
+          let errMsg = "Max iterations (" <> tshow maxIter <> ") reached"
+          engineOnError engineCfg errMsg
+          pure <| Left errMsg
+      | otherwise = do
+          let guardrailViolation = findGuardrailViolation guardrails' totalCost totalTokens toolCallCounts testFailures editFailures
+          case guardrailViolation of
+            Just (g, errMsg) -> do
+              engineOnGuardrail engineCfg g
+              pure <| Left errMsg
+            Nothing -> do
+              engineOnActivity engineCfg <| "Iteration " <> tshow (iteration + 1)
+              result <- Provider.chatWithUsage prov toolApis' msgs
+              case result of
+                Left err -> do
+                  engineOnError engineCfg err
+                  pure (Left err)
+                Right chatRes -> do
+                  let msg = Provider.chatMessage chatRes
+                      tokens = maybe 0 Provider.usageTotalTokens (Provider.chatUsage chatRes)
+                      cost = case Provider.chatUsage chatRes +> Provider.usageCost of
+                        Just actualCost -> actualCost * 100
+                        Nothing -> estimateCost (getProviderModel prov) tokens
+                  engineOnCost engineCfg tokens cost
+                  let newTokens = totalTokens + tokens
+                      newCost = totalCost + cost
+                  let assistantText = Provider.msgContent msg
+                  unless (Text.null assistantText)
+                    <| engineOnAssistant engineCfg assistantText
+                  case Provider.msgToolCalls msg of
+                    Nothing -> do
+                      engineOnActivity engineCfg "Agent completed"
+                      engineOnComplete engineCfg
+                      pure
+                        <| Right
+                        <| AgentResult
+                          { resultFinalMessage = Provider.msgContent msg,
+                            resultToolCallCount = totalCalls,
+                            resultIterations = iteration + 1,
+                            resultTotalCost = newCost,
+                            resultTotalTokens = newTokens
+                          }
+                    Just [] -> do
+                      engineOnActivity engineCfg "Agent completed (empty tool calls)"
+                      engineOnComplete engineCfg
+                      pure
+                        <| Right
+                        <| AgentResult
+                          { resultFinalMessage = Provider.msgContent msg,
+                            resultToolCallCount = totalCalls,
+                            resultIterations = iteration + 1,
+                            resultTotalCost = newCost,
+                            resultTotalTokens = newTokens
+                          }
+                    Just tcs -> do
+                      (toolResults, newTestFailures, newEditFailures) <- executeProviderToolCalls engineCfg toolMap tcs testFailures editFailures
+                      let newMsgs = msgs <> [msg] <> toolResults
+                          newCalls = totalCalls + length tcs
+                          newToolCallCounts = updateProviderToolCallCounts toolCallCounts tcs
+                      loopProvider prov toolApis' toolMap newMsgs (iteration + 1) newCalls newTokens newCost newToolCallCounts newTestFailures newEditFailures
+
+    getProviderModel :: Provider.Provider -> Text
+    getProviderModel (Provider.OpenRouter cfg) = Provider.providerModel cfg
+    getProviderModel (Provider.Ollama cfg) = Provider.providerModel cfg
+    getProviderModel (Provider.AmpCLI _) = "amp"
+
+    updateProviderToolCallCounts :: Map.Map Text Int -> [Provider.ToolCall] -> Map.Map Text Int
+    updateProviderToolCallCounts =
+      foldr (\tc m -> Map.insertWith (+) (Provider.fcName (Provider.tcFunction tc)) 1 m)
+
+    executeProviderToolCalls :: EngineConfig -> Map.Map Text Tool -> [Provider.ToolCall] -> Int -> Int -> IO ([Provider.Message], Int, Int)
+    executeProviderToolCalls eCfg tMap tcs initialTestFailures initialEditFailures = do
+      results <- traverse (executeSingleProvider eCfg tMap) tcs
+      let msgs = map (\(m, _, _) -> m) results
+          testDeltas = map (\(_, t, _) -> t) results
+          editDeltas = map (\(_, _, e) -> e) results
+          totalTestFail = initialTestFailures + sum testDeltas
+          totalEditFail = initialEditFailures + sum editDeltas
+      pure (msgs, totalTestFail, totalEditFail)
+
+    executeSingleProvider :: EngineConfig -> Map.Map Text Tool -> Provider.ToolCall -> IO (Provider.Message, Int, Int)
+    executeSingleProvider eCfg tMap tc = do
+      let name = Provider.fcName (Provider.tcFunction tc)
+          argsText = Provider.fcArguments (Provider.tcFunction tc)
+          callId = Provider.tcId tc
+      engineOnActivity eCfg <| "Executing tool: " <> name
+      engineOnToolCall eCfg name argsText
+      case Map.lookup name tMap of
+        Nothing -> do
+          let errMsg = "Tool not found: " <> name
+          engineOnToolResult eCfg name False errMsg
+          pure (Provider.Message Provider.ToolRole errMsg Nothing (Just callId), 0, 0)
+        Just tool -> do
+          case Aeson.decode (BL.fromStrict (TE.encodeUtf8 argsText)) of
+            Nothing -> do
+              let errMsg = "Invalid JSON arguments: " <> argsText
+              engineOnToolResult eCfg name False errMsg
+              pure (Provider.Message Provider.ToolRole errMsg Nothing (Just callId), 0, 0)
+            Just args -> do
+              resultValue <- toolExecute tool args
+              let resultText = TE.decodeUtf8 (BL.toStrict (Aeson.encode resultValue))
+                  isTestCall = name == "bash" && ("bild --test" `Text.isInfixOf` argsText || "bild -t" `Text.isInfixOf` argsText)
+                  isTestFailure = isTestCall && isFailureResultProvider resultValue
+                  testDelta = if isTestFailure then 1 else 0
+                  isEditFailure = name == "edit_file" && isOldStrNotFoundProvider resultValue
+                  editDelta = if isEditFailure then 1 else 0
+              engineOnToolResult eCfg name True resultText
+              pure (Provider.Message Provider.ToolRole resultText Nothing (Just callId), testDelta, editDelta)
+
+    isFailureResultProvider :: Aeson.Value -> Bool
+    isFailureResultProvider (Aeson.Object obj) =
+      case KeyMap.lookup "exit_code" obj of
+        Just (Aeson.Number n) -> n /= 0
+        _ -> False
+    isFailureResultProvider (Aeson.String s) =
+      "error"
+        `Text.isInfixOf` Text.toLower s
+        || "failed"
+        `Text.isInfixOf` Text.toLower s
+        || "FAILED"
+        `Text.isInfixOf` s
+    isFailureResultProvider _ = False
+
+    isOldStrNotFoundProvider :: Aeson.Value -> Bool
+    isOldStrNotFoundProvider (Aeson.Object obj) =
+      case KeyMap.lookup "error" obj of
+        Just (Aeson.String s) -> "old_str not found" `Text.isInfixOf` s
+        _ -> False
+    isOldStrNotFoundProvider _ = False