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/core-skills/tests/environment_loader.rs

@@ -469,7 +469,7 @@ async fn host_loading_reuses_walk_inventory_for_symlinked_skill_pack() {
             path: host_root.abs(),
             scope: SkillScope::User,
             file_system,
-            plugin_id: None,
+            plugin_identity: None,
             plugin_namespace: None,
             plugin_root: None,
             discovery_mode: codex_utils_plugins::SkillDiscoveryMode::Recursive,
@@ -504,6 +504,7 @@ async fn host_loading_reuses_walk_inventory_for_symlinked_skill_pack() {
                 path_to_skills_md: first_skill_path,
                 scope: SkillScope::User,
                 plugin_id: None,
+                remote_plugin_id: None,
             },
             SkillMetadata {
                 name: "linked:second".to_string(),
@@ -515,6 +516,7 @@ async fn host_loading_reuses_walk_inventory_for_symlinked_skill_pack() {
                 path_to_skills_md: second_skill_path,
                 scope: SkillScope::User,
                 plugin_id: None,
+                remote_plugin_id: None,
             },
         ]
     );