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>

docs/escalation.md

@@ -225,7 +225,7 @@ EOF
 gt escalate --type decision --issue $ISSUE "Caching approach needs decision"
 
 # 3. Exit cleanly
-gt done --exit ESCALATED
+gt done --status ESCALATED
 ```
 
 ## Mayor Startup Check