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>

steward/alerts/service.py

@@ -28,7 +28,7 @@ CORE_METRICS: dict[str, list[str]] = {
     "dns":     ["is_up", "response_ms"],
     "http":    ["is_up", "response_ms"],
     "steward": ["ingest_posts", "ingest_failed", "ingest_slowest_ms", "db_roundtrip_ms",
-                "unwatched_secs", "cleanup_secs", "open_fds", "open_fds_pct",
+                "unwatched_secs", "restart_gap_secs", "cleanup_secs", "open_fds", "open_fds_pct",
                 "backup_age_secs", "backup_bytes"],
 }