Fix session-scoped agent status updates

[cbusillo]

Summary

• keep a separate agent status event sender per active session instead of replacing the previous sender
• tag created agents with the owning session and send each TUI only that session's agent status payload
• constrain gh_run_wait run selection by optional head_sha so post-merge waits do not attach to another session's newer run on the same branch

Refs #175

Validation

• ./build-fast.sh