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>

steward/mcp/tools/alerts.py

@@ -204,13 +204,13 @@ async def get_alert_rule_options(source_module: str = "") -> dict:
     resource: is_up (1 up, 0 down) and response_ms.
     """
     async with db_session() as db:
+        sources = alert_svc.source_modules()
         if not source_module:
-            return {"sources": {src: await alert_svc.metrics_for(db, src)
-                                for src in alert_svc.SOURCE_MODULES},
+            return {"sources": {src: await alert_svc.metrics_for(db, src) for src in sources},
                     "operators": [o.value for o in AlertOperator]}
-        if source_module not in alert_svc.METRIC_CATALOG:
+        if source_module not in sources:
             raise ValueError(f"source_module {source_module!r} is not one of "
-                             f"{', '.join(alert_svc.SOURCE_MODULES)}.")
+                             f"{', '.join(sources)}.")
         return {"source_module": source_module,
                 "metrics": await alert_svc.metrics_for(db, source_module),
                 "resources": await alert_svc.resources_for(db, source_module),