fix(mcp): get_anomaly shows Steward's own restarts beside every anomaly, and a router's containers (#5862)

All 16 Traefik 4xx anomalies on 2026-10-15 came back with nothing
around them, though 15 were Steward's own restarts. A router has no
host, and every events source was read by host.

- Core: each break of more than 90 s in Steward's once-a-minute health
  sample is a "steward" event beside every anomaly, placed or not.
- Plugins: an optional mcp_resource_events(db, source, resource, since,
  until) for a resource with no host. Traefik's gives the container
  events of the swarm service behind a router, on any host.

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

docs/core/mcp.md

@@ -159,6 +159,8 @@ exposes `register_mcp_tools(register)`, optionally `mcp_overview(db)` to
 contribute findings to `get_overview`, and optionally
 `mcp_events(db, host_id, host_name, since, until)`: what it knows happened on a
 host in a window, as a list of `steward.mcp.tools._events.event(...)`, which
-`get_anomaly` shows beside an anomaly. See
+`get_anomaly` shows beside an anomaly; for a resource with no host, optionally
+`mcp_resource_events(db, source, resource, since, until)`. Core adds the
+stretches Steward itself wasn't recording to every anomaly. See
 [Writing a plugin](../plugins/writing-a-plugin.md). A tool that changes
 anything calls `audit_mcp(...)` from `steward/mcp/tools/_audit.py`.