Add agent observability: event logging and storage

- Add Omni/Agent/Event.hs with AgentEvent types
- Add agent_events table schema and CRUD functions to Core.hs
- Add new callbacks to Engine.hs: onAssistant, onToolResult, onComplete, onError
- Wire event logging into Worker.hs with session tracking

Events are now persisted to SQLite for each agent work session,
enabling visibility into agent reasoning and tool usage.

Task-Id: t-197.1
Task-Id: t-197.2
Task-Id: t-197.3

1 files changed, 115 insertions, 0 deletions

diff --git a/Omni/Task/Core.hs b/Omni/Task/Core.hs
index c930b2c..699e4f3 100644
--- a/Omni/Task/Core.hs
+++ b/Omni/Task/Core.hs
@@ -508,6 +508,23 @@ runMigrations conn = do
   migrateTable conn "tasks" tasksColumns
   migrateTable conn "retry_context" retryContextColumns
   migrateTable conn "facts" factsColumns
+  createAgentEventsTable conn
+
+-- | Create agent_events table if it doesn't exist
+createAgentEventsTable :: SQL.Connection -> IO ()
+createAgentEventsTable conn = do
+  SQL.execute_
+    conn
+    "CREATE TABLE IF NOT EXISTS agent_events (\
+    \ id INTEGER PRIMARY KEY AUTOINCREMENT, \
+    \ task_id TEXT NOT NULL, \
+    \ session_id TEXT NOT NULL, \
+    \ timestamp DATETIME DEFAULT CURRENT_TIMESTAMP, \
+    \ event_type TEXT NOT NULL, \
+    \ content TEXT NOT NULL \
+    \)"
+  SQL.execute_ conn "CREATE INDEX IF NOT EXISTS idx_agent_events_task ON agent_events(task_id)"
+  SQL.execute_ conn "CREATE INDEX IF NOT EXISTS idx_agent_events_session ON agent_events(session_id)"
 
 -- | Expected columns for task_activity table (name, type, nullable)
 taskActivityColumns :: [(Text, Text)]
@@ -1565,3 +1582,101 @@ deleteFact :: Int -> IO ()
 deleteFact fid =
   withDb <| \conn ->
     SQL.execute conn "DELETE FROM facts WHERE id = ?" (SQL.Only fid)
+
+-- ============================================================================
+-- Agent Events (for observability)
+-- ============================================================================
+
+-- | Stored agent event record
+data StoredEvent = StoredEvent
+  { storedEventId :: Int,
+    storedEventTaskId :: Text,
+    storedEventSessionId :: Text,
+    storedEventTimestamp :: UTCTime,
+    storedEventType :: Text,
+    storedEventContent :: Text
+  }
+  deriving (Show, Eq, Generic)
+
+instance ToJSON StoredEvent
+
+instance FromJSON StoredEvent
+
+instance SQL.FromRow StoredEvent where
+  fromRow =
+    StoredEvent
+      </ SQL.field
+      <*> SQL.field
+      <*> SQL.field
+      <*> SQL.field
+      <*> SQL.field
+      <*> SQL.field
+
+-- | Generate a new session ID (timestamp-based for simplicity)
+generateSessionId :: IO Text
+generateSessionId = do
+  now <- getCurrentTime
+  pure <| "s-" <> T.pack (show now)
+
+-- | Insert an agent event
+insertAgentEvent :: Text -> Text -> Text -> Text -> IO ()
+insertAgentEvent taskId sessionId eventType content =
+  withDb <| \conn ->
+    SQL.execute
+      conn
+      "INSERT INTO agent_events (task_id, session_id, event_type, content) VALUES (?, ?, ?, ?)"
+      (taskId, sessionId, eventType, content)
+
+-- | Get all events for a task (most recent session)
+getEventsForTask :: Text -> IO [StoredEvent]
+getEventsForTask taskId = do
+  maybeSession <- getLatestSessionForTask taskId
+  case maybeSession of
+    Nothing -> pure []
+    Just sid -> getEventsForSession sid
+
+-- | Get all events for a specific session
+getEventsForSession :: Text -> IO [StoredEvent]
+getEventsForSession sessionId =
+  withDb <| \conn ->
+    SQL.query
+      conn
+      "SELECT id, task_id, session_id, timestamp, event_type, content \
+      \FROM agent_events WHERE session_id = ? ORDER BY id ASC"
+      (SQL.Only sessionId)
+
+-- | Get all sessions for a task
+getSessionsForTask :: Text -> IO [Text]
+getSessionsForTask taskId =
+  withDb <| \conn -> do
+    rows <-
+      SQL.query
+        conn
+        "SELECT DISTINCT session_id FROM agent_events WHERE task_id = ? ORDER BY session_id DESC"
+        (SQL.Only taskId) ::
+        IO [SQL.Only Text]
+    pure [sid | SQL.Only sid <- rows]
+
+-- | Get the most recent session ID for a task
+getLatestSessionForTask :: Text -> IO (Maybe Text)
+getLatestSessionForTask taskId =
+  withDb <| \conn -> do
+    rows <-
+      SQL.query
+        conn
+        "SELECT session_id FROM agent_events WHERE task_id = ? ORDER BY id DESC LIMIT 1"
+        (SQL.Only taskId) ::
+        IO [SQL.Only Text]
+    pure <| case rows of
+      [SQL.Only sid] -> Just sid
+      _ -> Nothing
+
+-- | Get events for a task since a given event ID (for streaming/polling)
+getEventsSince :: Text -> Int -> IO [StoredEvent]
+getEventsSince sessionId lastId =
+  withDb <| \conn ->
+    SQL.query
+      conn
+      "SELECT id, task_id, session_id, timestamp, event_type, content \
+      \FROM agent_events WHERE session_id = ? AND id > ? ORDER BY id ASC"
+      (sessionId, lastId)