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

0 files changed, 0 insertions, 0 deletions