fix(mcp): get_anomaly shows a quick Steward restart, not only a gap over 90 s (#5862)

A redeploy's new process ticks at once, so its once-a-minute samples never
broke for 90 s and get_anomaly showed none of the day's restarts. The first
self-monitor tick in a process now records restart_gap_secs (time since the
last process's last sample), and steward_gaps shows each one as a restart.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/core/alerting.md

@@ -138,6 +138,7 @@ failed agent posts, a gap in watching, or open files near the limit.
 | `ingest_slowest_ms` | The slowest post in the last minute; agents give up at 10 s |
 | `db_roundtrip_ms` | A `SELECT 1`, connection checkout included |
 | `unwatched_secs` | The gap before this tick when it exceeds `watch.GAP_S` (a deploy or an outage); 0 when unbroken |
+| `restart_gap_secs` | Recorded once, on a new process's first tick: the time since the last process's last sample, however short |
 | `cleanup_secs` | How long the last hourly retention and rollup run took |
 | `open_fds` / `open_fds_pct` | Steward's open files, and as a share of its limit (absent without `/proc`) |