Integrate event logging into Engine.hs

Update Engine.hs to emit events via callbacks:

1. Add new callbacks to EngineConfig:

• engineOnAssistant :: Text -> IO ()
• engineOnToolCall :: Text -> Value -> IO ()
• engineOnToolResult :: Text -> Bool -> Text -> IO ()
• engineOnComplete :: IO ()
• engineOnError :: Text -> IO ()

2. Call these at appropriate points in runAgent loop:

• After receiving assistant message
• Before executing each tool call
• After each tool result
• On completion or error

3. Update Worker.hs to wire callbacks to EventStore.insertEvent

The callbacks should be optional (default to no-op) to avoid breaking existing usage.