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/utils/plugins/src/lib.rs

@@ -26,10 +26,17 @@ pub enum SkillDiscoveryMode {
     DirectChildren,
 }
 
+/// The local identifier and optional remote identifier for a plugin.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct PluginIdentity {
+    pub plugin_id: String,
+    pub remote_plugin_id: Option<String>,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct PluginSkillRoot {
     pub path: AbsolutePathBuf,
-    pub plugin_id: String,
+    pub plugin_identity: PluginIdentity,
     pub plugin_namespace: String,
     pub plugin_root: AbsolutePathBuf,
     pub discovery_mode: SkillDiscoveryMode,