feat(lifecycle): immediate daemon notification via SIGUSR1

- daemon.go: Add SIGUSR1 handler to process lifecycle requests immediately
- handoff.go: Signal daemon after sending lifecycle mail to deacon/
- mail.go: Show message IDs in hook output for direct reading

Previously, gt handoff would wait up to 5 min for daemon heartbeat poll.
Now lifecycle requests are processed within milliseconds.

Closes gt-zut3

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

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

internal/cmd/mail.go

@@ -597,7 +597,7 @@ func runMailCheck(cmd *cobra.Command, args []string) error {
 			messages, _ := mailbox.ListUnread()
 			var subjects []string
 			for _, msg := range messages {
-				subjects = append(subjects, fmt.Sprintf("- From %s: %s", msg.From, msg.Subject))
+				subjects = append(subjects, fmt.Sprintf("- %s from %s: %s", msg.ID, msg.From, msg.Subject))
 			}
 
 			fmt.Println("<system-reminder>")