Propagate remote plugin IDs to skill metadata (#35261)

## What changed

- Carry a plugin's local and remote identities together from plugin loading into
  `SkillMetadata`.
- Resolve remote IDs from the installed-plugin snapshot when available, falling
  back to persisted install metadata only when no snapshot exists.
- Include plugin identity in skill cache keys so identity changes refresh cached
  skill metadata.

## Testing

- Cover snapshot and persisted identity resolution, local marketplace isolation,
  cached skill refreshes, and propagation through plugin skill snapshots.

GitOrigin-RevId: aabeeb631a43361fe817358ace7f1ea8ba5db708

codex-rs/ext/skills/src/provider/host_tests.rs

@@ -36,7 +36,7 @@ async fn host_catalog_entries_carry_their_render_metadata() -> Result<(), Box<dy
             path: root.clone(),
             scope: SkillScope::User,
             file_system: Arc::clone(&LOCAL_FS),
-            plugin_id: None,
+            plugin_identity: None,
             plugin_namespace: None,
             plugin_root: None,
             discovery_mode: Default::default(),