feat(spawn): Replace tmux injection with persistent mail-based work assignment (ga-yp3)

- gt spawn now sends work assignment to polecat inbox instead of tmux injection
- Add --identity flag to gt mail inbox and gt mail check
- Add --force flag to gt spawn to override existing unread mail
- Update polecat template with startup protocol for reading inbox
- Fix pre-existing lint issue in start.go

The new flow is more reliable:
1. Spawn sends work assignment mail to polecat inbox
2. Polecat starts and runs gt prime
3. gt prime automatically runs gt mail check --inject
4. Polecat reads work assignment from inbox

Benefits:
- Persistence across session restarts
- No racing against Claude initialization
- Audit trail in beads
- Edge case handling for existing unread mail

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

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

internal/cmd/start.go

@@ -203,7 +203,7 @@ func runGracefulShutdown(t *tmux.Tmux) error {
 }
 
 func runImmediateShutdown(t *tmux.Tmux) error {
-	fmt.Println("Shutting down Gas Town...\n")
+	fmt.Println("Shutting down Gas Town...")
 
 	stopped := 0