t-354.5: Consolidate Engine.hs - delete dead runtime code

Consolidate Engine.hs - extract shared types, delete dead runtime code.

Engine.hs is ~1700 lines but most of the runtime code (runAgent*, chat) is now dead - everything uses Op + Sequential interpreter instead.

Changes: 1. Keep shared types: Tool, Message, Role, ToolCall, ToolResult, etc. 2. Delete dead runtime functions: runAgent, runAgentWithProvider, runAgentWithProviderStreaming, chat 3. Consider moving types to Omni/Agent/Types.hs or keeping in Engine.hs as type library 4. Update any remaining imports

Verify by:

• grep for Engine.runAgent* calls (should be none in runtime code)
• Build all consumers after changes