fix(resource-registry): implement get_parents() on ResourceDagMixin

Add get_parents(name_or_id: str) -> list[Resource] method to ResourceDagMixin
in _resource_registry_dag.py. The method is symmetric to the existing
get_children() method but queries ResourceLinkModel by child_id instead of
parent_id to return all direct parent resources.

Implementation details:
- Calls show_resource() first to raise NotFoundError for non-existent resources
- Queries ResourceLinkModel filtered by child_id=resource.resource_id
- Returns deterministically sorted list (by name, then resource_id)
- Exposed via ResourceRegistryService through the mixin inheritance chain

The BDD scenario 'Get parents returns all direct parents' in
resource_dag.feature now passes. All 15 existing @dag_traversal and other
DAG scenarios continue to pass with no regression.

ISSUES CLOSED: #2844

src/cleveragents/application/services/_resource_registry_dag.py

@@ -226,6 +226,41 @@ class ResourceDagMixin:
         finally:
             session.close()
 
+    def get_parents(self: RegistryHost, name_or_id: str) -> list[Resource]:
+        """Get direct parents of a resource.
+
+        Args:
+            name_or_id: Resource name or ULID.
+
+        Returns:
+            List of parent ``Resource`` domain objects, sorted by name.
+
+        Raises:
+            NotFoundError: If the resource does not exist.
+        """
+        resource = self.show_resource(name_or_id)
+        session = self._session()
+        try:
+            links = (
+                session.query(ResourceLinkModel)
+                .filter_by(child_id=resource.resource_id)
+                .all()
+            )
+            parents: list[Resource] = []
+            for link in links:
+                parent_row = (
+                    session.query(ResourceModel)
+                    .filter_by(resource_id=str(link.parent_id))
+                    .first()
+                )
+                if parent_row is not None:
+                    parents.append(db_resource_to_domain(parent_row))
+            # Deterministic ordering: sort by name, then resource_id
+            parents.sort(key=lambda r: (r.name or "", r.resource_id))
+            return parents
+        finally:
+            session.close()
+
     def get_resource_tree(
         self: RegistryHost,
         name_or_id: str,