feat(control): add agent_end event subscription and wait support

[kylerky]

Summary

Adds agent_end event support to the control extension, alongside the existing turn_end event.

Changes

• Subscribe command now accepts "agent_end" as an event type (alongside "turn_end")
• agent_end event handler fires to all subscribers when the agent loop completes — this spans multiple turns,
  unlike turn_end which fires after each individual turn
• wait_until parameter for send actions now accepts "agent_end" (in addition to "turn_end" and
  "message_processed")
• CLI bridge (--send-session-wait) supports agent_end as a wait mode
• Subscribe-before-send ordering fix: the subscription is now registered before the command is sent, ensuring the
  event is always caught regardless of timing

Why

turn_end fires after each AI turn, but sometimes callers want to wait for the entire agent loop to finish (which may
involve multiple turns with tool calls). agent_end signals true completion of the agent processing cycle.