feat: secure build phase 2 — enforce image verification by default

All store images are now digest-pinned and cosign-signed by the publish
pipeline, so the warn-by-default training-wheels period is over: an
unsigned or undigested image must not install unless the admin
explicitly opts out. The service_composer fallback used when the config
manager is unavailable or corrupt also flips to enforce — config
corruption must fail closed rather than silently weaken verification.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_config_manager.py

@@ -518,7 +518,7 @@ class TestEmailManagerApply(unittest.TestCase):
 
 
 class TestImageVerificationConfig(unittest.TestCase):
-    """image_verification config round-trip and warn-by-default behaviour."""
+    """image_verification config round-trip; default is now 'enforce' (P3)."""
 
     def setUp(self):
         self.temp_dir = tempfile.mkdtemp()
@@ -530,9 +530,9 @@ class TestImageVerificationConfig(unittest.TestCase):
     def tearDown(self):
         shutil.rmtree(self.temp_dir)
 
-    def test_default_mode_is_warn(self):
-        self.assertEqual(self.cm.get_image_verification_mode(), 'warn')
-        self.assertEqual(self.cm.get_image_verification(), {'mode': 'warn'})
+    def test_default_mode_is_enforce(self):
+        self.assertEqual(self.cm.get_image_verification_mode(), 'enforce')
+        self.assertEqual(self.cm.get_image_verification(), {'mode': 'enforce'})
 
     def test_set_and_get_round_trip(self):
         for mode in ('off', 'warn', 'enforce'):
@@ -548,9 +548,9 @@ class TestImageVerificationConfig(unittest.TestCase):
         with self.assertRaises(ValueError):
             self.cm.set_image_verification_mode('paranoid')
 
-    def test_corrupt_section_falls_back_to_warn(self):
+    def test_corrupt_section_falls_back_to_enforce(self):
         self.cm.configs['image_verification'] = {'mode': 'bogus'}
-        self.assertEqual(self.cm.get_image_verification_mode(), 'warn')
+        self.assertEqual(self.cm.get_image_verification_mode(), 'enforce')
 
 
 if __name__ == '__main__':