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/plugins/writing-a-plugin.md

@@ -408,7 +408,7 @@ async def mcp_events(db, host_id, host_name, since, until) -> list[dict]:
 
 A resource with no host (a Traefik router, say) has no host to read events by. A plugin that knows what happened to one of its own resources exports `mcp_resource_events(db, source, resource, since, until)` instead, same return shape, and returns `[]` for a source that isn't its own. Traefik's is the example: it gives a router's swarm service's container events, on whichever hosts they ran.
 
-Core adds, beside every anomaly, the stretches Steward itself wasn't recording (kind `"steward"`: a restart, a deploy, a stall), read from its own once-a-minute health sample. A plugin doesn't need to report those.
+Core adds, beside every anomaly, the stretches Steward itself wasn't recording (kind `"steward"`: a restart, a deploy, a stall), read from its own once-a-minute health sample; a restart is shown however quick, from the `restart_gap_secs` its new process records. A plugin doesn't need to report those.
 
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