feat(ci): manifest-aware dependency PRs and cleanup protection

Implements: DEVX-165 REQ-1..3

- create_dependency_pr gains --manifest/--verify-container/--source-ref:
  resolves the producer image's OCI digest via the packages API before
  opening the PR, then writes structured manifest fields
  (version/git_ref/image_tag/image_digest/updated_at) in the PR branch
- clean_images gains --protect: pinned versions survive --keep trimming
  so a manifest-referenced digest can't be garbage-collected
- regression tests for digest resolution, manifest helpers, verify
  ordering, and protect filtering

tests/unit/test_build_image.py

@@ -485,6 +485,38 @@ class TestSelectForDeletion:
         to_delete = select_for_deletion(versions, keep=5)
         assert len(to_delete) == 0
 
+    def test_protect_exempts_version(self) -> None:
+        """REQ-2: a manifest-pinned version survives cleanup beyond --keep."""
+        versions = [
+            {"version": "0.1.0", "created_at": "2025-01-01"},
+            {"version": "0.2.0", "created_at": "2025-02-01"},
+            {"version": "0.3.0", "created_at": "2025-03-01"},
+            {"version": "0.4.0", "created_at": "2025-04-01"},
+        ]
+        to_delete = select_for_deletion(versions, keep=2, protect=frozenset({"0.1.0"}))
+        deleted = {v["version"] for v in to_delete}
+        assert "0.1.0" not in deleted
+        assert deleted == {"0.2.0"}
+
+    def test_protect_multiple_versions(self) -> None:
+        versions = [
+            {"version": "0.1.0", "created_at": "2025-01-01"},
+            {"version": "0.2.0", "created_at": "2025-02-01"},
+            {"version": "0.3.0", "created_at": "2025-03-01"},
+            {"version": "0.4.0", "created_at": "2025-04-01"},
+        ]
+        to_delete = select_for_deletion(versions, keep=1, protect=frozenset({"0.1.0", "0.2.0"}))
+        assert {v["version"] for v in to_delete} == {"0.3.0"}
+
+    def test_protect_default_empty_behaves_as_before(self) -> None:
+        versions = [
+            {"version": "0.1.0", "created_at": "2025-01-01"},
+            {"version": "0.2.0", "created_at": "2025-02-01"},
+            {"version": "0.3.0", "created_at": "2025-03-01"},
+        ]
+        to_delete = select_for_deletion(versions, keep=2)
+        assert {v["version"] for v in to_delete} == {"0.1.0"}
+
 
 class TestCleanImagesAPI:
     """Tests for the clean_images module's API functions."""