Polecats now fully clean up after themselves on `gt done`:
- Step 1: Nuke worktree (existing behavior)
- Step 2: Kill own tmux session (new)
This completes the "done means gone" model - both worktree and
session are terminated. Previously the session survived as a zombie.
Audit logging added to both systems:
- townlog: EventKill for `gt log` visibility
- events: TypeSessionDeath with structured payload
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
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