Support packaged defaults in config layering (#37810)

## What changed

- Load an optional package-supplied config file as the lowest-precedence
  configuration layer, so every existing layer can override its values.
- Report the layer as `packagedDefaults`, including its source path, through
  config diagnostics and the app-server protocol.
- Return an error when a configured packaged defaults file is missing.

## Testing

- Cover precedence across packaged, system, user, and session layers.
- Cover the missing-file error path.

GitOrigin-RevId: 52406890f4d580eedabc90bf2a9ee3affa36fee1

codex-rs/ext/skills/src/host_roots.rs

@@ -117,7 +117,8 @@ fn roots_from_layer_stack(
                     SkillScope::Admin,
                 ));
             }
-            ConfigLayerSource::Mdm { .. }
+            ConfigLayerSource::PackagedDefaults { .. }
+            | ConfigLayerSource::Mdm { .. }
             | ConfigLayerSource::EnterpriseManaged { .. }
             | ConfigLayerSource::SessionFlags
             | ConfigLayerSource::LegacyManagedConfigTomlFromFile { .. }