fix(gateway): drain-aware hermes update + faster still-working pings (#14736)

cmd_update no longer SIGKILLs in-flight agent runs, and users get
'still working' status every 3 min instead of 10. Two long-standing
sources of '@user — agent gives up mid-task' reports on Telegram and
other gateways.

Drain-aware update:
- New helper hermes_cli.gateway._graceful_restart_via_sigusr1(pid,
  drain_timeout) sends SIGUSR1 to the gateway and polls os.kill(pid,
  0) until the process exits or the budget expires.
- cmd_update's systemd loop now reads MainPID via 'systemctl show
  --property=MainPID --value' and tries the graceful path first. The
  gateway's existing SIGUSR1 handler -> request_restart(via_service=
  True) -> drain -> exit(75) is wired in gateway/run.py and is
  respawned by systemd's Restart=on-failure (and the explicit
  RestartForceExitStatus=75 on newer units).
- Falls back to 'systemctl restart' when MainPID is unknown, the
  drain budget elapses, or the unit doesn't respawn after exit (older
  units missing Restart=on-failure). Old install behavior preserved.
- Drain budget = max(restart_drain_timeout, 30s) + 15s margin so the
  drain loop in run_agent + final exit have room before fallback
  fires. Composes with #14728's tool-subprocess reaping.

Notification interval:
- agent.gateway_notify_interval default 600 -> 180.
- HERMES_AGENT_NOTIFY_INTERVAL env-var fallback in gateway/run.py
  matched.
- 9-minute weak-model spinning runs now ping at 3 min and 6 min
  instead of 27 seconds before completion, removing the 'is the bot
  dead?' reflex that drives gateway-restart cycles.

Tests:
- Two new tests in tests/hermes_cli/test_update_gateway_restart.py:
  one asserts SIGUSR1 is sent and 'systemctl restart' is NOT called
  when MainPID is known and the helper succeeds; one asserts the
  fallback fires when the helper returns False.
- E2E: spawned detached bash processes confirm the helper returns
  True on SIGUSR1-handling exit (~0.5s) and False on SIGUSR1-ignoring
  processes (timeout). Verified non-existent PID and pid=0 edge cases.
- 41/41 in test_update_gateway_restart.py (was 39, +2 new).
- 154/154 in shutdown-related suites including #14728's new tests.

Reported by @GeoffWellman and @ANT_1515 on X.

gateway/run.py

@@ -10373,9 +10373,9 @@ class GatewayRunner:
         # Periodic "still working" notifications for long-running tasks.
         # Fires every N seconds so the user knows the agent hasn't died.
         # Config: agent.gateway_notify_interval in config.yaml, or
-        # HERMES_AGENT_NOTIFY_INTERVAL env var.  Default 600s (10 min).
+        # HERMES_AGENT_NOTIFY_INTERVAL env var.  Default 180s (3 min).
         # 0 = disable notifications.
-        _NOTIFY_INTERVAL_RAW = float(os.getenv("HERMES_AGENT_NOTIFY_INTERVAL", 600))
+        _NOTIFY_INTERVAL_RAW = float(os.getenv("HERMES_AGENT_NOTIFY_INTERVAL", 180))
         _NOTIFY_INTERVAL = _NOTIFY_INTERVAL_RAW if _NOTIFY_INTERVAL_RAW > 0 else None
         _notify_start = time.time()