Implement mcp purge command for registry cleanup

Add PurgeComponent RPC to the agent service that removes stale registry
entries for components that are both gone (observed state is removed,
unknown, or exited) and unwanted (not in any current service definition).
Refuses to purge components with running or stopped containers. When all
components of a service are purged, the service row is deleted too.
Supports --dry-run to preview without modifying the database.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/registry/events.go

@@ -83,6 +83,15 @@ func CountEvents(db *sql.DB, service, component string, since time.Time) (int, e
 	return count, nil
 }
 
+// DeleteComponentEvents deletes all events for a specific component.
+func DeleteComponentEvents(db *sql.DB, service, component string) error {
+	_, err := db.Exec("DELETE FROM events WHERE service = ? AND component = ?", service, component)
+	if err != nil {
+		return fmt.Errorf("delete events %q/%q: %w", service, component, err)
+	}
+	return nil
+}
+
 // PruneEvents deletes events older than the given time.
 func PruneEvents(db *sql.DB, before time.Time) (int64, error) {
 	res, err := db.Exec(