feat(alerts): the metric catalog is written by what emits it (#5061)

#5061 "Metric catalog belongs to what emits: plugins register their
metrics; names match what is recorded".

- The static METRIC_CATALOG is gone. metric_catalog() is core's own sources
  (the four monitor types and "steward", the app's open_fds/open_fds_pct,
  which were recorded but never offered) plus each loaded plugin's
  get_alert_metrics() export. A disabled plugin's sources leave the picker.
- traefik offers what it records: error_rate_4xx_pct and error_rate_5xx_pct.
  The old "error_rate" was never recorded, so a rule on it could not fire.
- tests/core/test_metric_catalog.py walks every MetricBatch.add /
  record_metric call (traefik's tuple loop included) and fails on a recorded
  metric the catalog lacks; it shows on a synthetic writer that it can fail.

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

plugins/host_agent/__init__.py

@@ -31,3 +31,13 @@ def register_mcp_tools(register) -> None:
 async def mcp_overview(db) -> list[dict]:
     from .mcp_tools import mcp_overview as findings
     return await findings(db)
+
+
+def get_alert_metrics() -> dict[str, list[str]]:
+    """Host metrics ingest records for alert rules (resource = the host name;
+    disk_used_pct per mount, resource "<host>:<mount>")."""
+    return {"host_agent": [
+        "cpu_pct", "mem_used_pct", "mem_available_bytes", "swap_used_bytes",
+        "disk_used_pct_worst", "disk_used_pct", "load_1m", "load_5m", "load_15m",
+        "uptime_secs",
+    ]}