feat(done): Add --exit flag for session self-termination (gt-lynar)

Add --exit flag to `gt done` that terminates the Claude session
immediately after submitting the MR to the merge queue. This prevents
polecats from sitting idle (and wasting money) while waiting for the
Witness to kill them.

Changes:
- Rename existing --exit flag to --status (for exit type)
- Add new --exit boolean flag for session self-termination
- Update docs and help text to reflect new flag names

Usage: gt done --exit

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/handoff.go

@@ -25,7 +25,7 @@ var handoffCmd = &cobra.Command{
 This is the canonical way to end any agent session. It handles all roles:
 
   - Mayor, Crew, Witness, Refinery, Deacon: Respawns with fresh Claude instance
-  - Polecats: Calls 'gt done --exit DEFERRED' (Witness handles lifecycle)
+  - Polecats: Calls 'gt done --status DEFERRED' (Witness handles lifecycle)
 
 When run without arguments, hands off the current session.
 When given a bead ID (gt-xxx, hq-xxx), hooks that work first, then restarts.