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-skills/src/injection.rs

@@ -99,6 +99,7 @@ pub async fn build_skill_injections(
                     skill_scope: skill.scope,
                     skill_path: skill.path_to_skills_md.to_path_buf(),
                     plugin_id: skill.plugin_id.clone(),
+                    remote_plugin_id: skill.remote_plugin_id.clone(),
                     invocation_type: InvocationType::Explicit,
                 });
                 result.items.push(SkillInjection {