feat: send PTY exit notifications to the session agent

[Llacoste2000]

Summary

• Persist the current OpenCode agent on PTY sessions so exit notifications keep the right request context.
• Thread the agent id through spawn/session lifecycle into the notification manager for deterministic routing.
• Add tests that verify notifications are sent using the originating agent for each PTY session.

Closes #23

[shekohex]

thanks! will publish a new version in a moment.