Support plugin roots in the host skill loader (#37267)

## What changed

- Carry plugin identity, namespace, root, and discovery mode through host skill loading.
- Apply the owning plugin namespace and IDs to loaded skill metadata.
- Respect direct-child discovery for Agent Plugins and reject skills that resolve outside the plugin root or are not regular files.
- Preserve recursive discovery and symlink behavior for legacy plugin roots, and allow the full 64-character namespace plus 64-character skill name.

## Testing

Add host-loader coverage for plugin metadata, namespace ownership, direct-child filtering, path containment, recursive symlinks, shared assets, and maximum-length qualified names.

GitOrigin-RevId: cdde821643ce39bd030d0c3753b3304b75161690

codex-rs/ext/skills/src/host_service.rs

@@ -303,12 +303,11 @@ impl HostSkillsService {
                 let snapshot = if use_legacy_loader {
                     load_skill_root_snapshot(root, plugin_skill_snapshots).await
                 } else {
-                    let snapshot = load_host_skill_root(HostSkillRoot {
-                        path: root.path,
-                        scope: root.scope,
-                        file_system: root.file_system,
-                        plugin_root: root.plugin_root,
-                    })
+                    let snapshot = load_host_skill_root(HostSkillRoot::host(
+                        root.path,
+                        root.scope,
+                        root.file_system,
+                    ))
                     .await;
                     SkillRootSnapshot::new(
                         snapshot.root,