Add plugin ID to skill analytics (#20923)

## Summary
- thread plugin skill roots through the skills loader with their plugin
ID
- store plugin ID on loaded skill metadata for plugin-provided skills
- include plugin ID on skill invocation analytics events

## Test plan
- cargo check -p codex-core-skills
- cargo check -p codex-core -p codex-core-plugins -p codex-analytics
- cargo check -p codex-tui
- cargo check -p codex-plugin -p codex-core -p codex-core-plugins -p
codex-analytics
- cargo check -p codex-app-server
- cargo test -p codex-analytics
- HOME=/private/tmp/codex-empty-home cargo test -p codex-core-skills
- just fix -p codex-core-skills
- just fix -p codex-analytics
- just fix -p codex-core-plugins
- just fix -p codex-core
- just fmt
- git diff --check

codex-rs/utils/plugins/src/lib.rs

@@ -1,9 +1,17 @@
 //! Plugin path resolution, plaintext mention sigils, and MCP connector helpers shared across Codex
 //! crates.
 
+use codex_utils_absolute_path::AbsolutePathBuf;
+
 pub mod mcp_connector;
 pub mod mention_syntax;
 pub mod plugin_namespace;
 
 pub use plugin_namespace::find_plugin_manifest_path;
 pub use plugin_namespace::plugin_namespace_for_skill_path;
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct PluginSkillRoot {
+    pub path: AbsolutePathBuf,
+    pub plugin_id: String,
+}