Track remote plugin IDs in skill invocation analytics (#35262)

## What changed

- Add `remote_plugin_id` to skill invocation facts and analytics event parameters.
- Propagate the ID for both explicit and implicit plugin skill invocations.

## Testing

- Cover explicit and implicit remote plugin skill invocations with analytics integration tests.

GitOrigin-RevId: 3e1d25b5ff88dcff9c2b6c9fe44bca97e8778221

codex-rs/core/src/skills.rs

@@ -66,6 +66,7 @@ pub(crate) async fn maybe_emit_implicit_skill_invocation(
         skill_scope: candidate.scope,
         skill_path: candidate.path_to_skills_md.to_path_buf(),
         plugin_id: candidate.plugin_id,
+        remote_plugin_id: candidate.remote_plugin_id,
         invocation_type: InvocationType::Implicit,
     };
     let skill_scope = match invocation.skill_scope {