Standardize session end with gt handoff (gt-yt6g)

- Update gt handoff to detect polecats (via GT_POLECAT env var) and
  call gt done instead of respawning - polecats have Witness-managed lifecycle
- Update CLAUDE.md Session End Checklist to use gt handoff
- This is the canonical way to end any agent session - it handles mail,
  respawn, and context restoration via SessionStart hook

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

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

CLAUDE.md

@@ -109,13 +109,17 @@ Issue prefix: `gt-`
 
 ```
 [ ] git status              (check for uncommitted changes)
-[ ] git push                (push any commits)
+[ ] git add && git commit   (commit any changes)
 [ ] bd sync                 (sync beads changes)
-[ ] Check inbox             (any messages needing response?)
-[ ] HANDOFF if incomplete:
-    gt mail send gastown/crew/joe -s "🤝 HANDOFF: ..." -m "..."
+[ ] git push                (push to remote - CRITICAL)
+[ ] gt handoff              (hand off to fresh session)
+    # Or with context: gt handoff -s "Brief" -m "Details"
 ```
 
+**Why `gt handoff`?** This is the canonical way to end your session. It handles
+everything: sends handoff mail, respawns with fresh context, and your work
+continues from where you left off via your pinned molecule.
+
 Crew member: joe
 Rig: gastown
 Working directory: /Users/stevey/gt/gastown/crew/joe