Three defenses against SIGTERM-respawn loops when agent schedules its
own gateway restart under launchd/systemd KeepAlive:
1. HERMES_IN_GATEWAY env var: gateway sets it at startup; stop/restart
subcommands refuse to run when set (exit 1 with clear message).
2. Cron create payload filter: regex pre-flight rejects prompts/scripts
containing hermes gateway restart/stop, launchctl kickstart/unload,
systemctl restart/stop, and pkill patterns.
3. 30 new tests: pattern matching (14), cron block (5), gateway guard (4),
safe command negatives (7).
