Load executor skills without host path conversion (#29626)

## Why

After #28918, selected skill roots are `PathUri`, but the executor skill
provider still converts them to the app-server host's `AbsolutePathBuf`.
A foreign Windows root therefore cannot be discovered by a Unix host,
and the inverse has the same problem.

This PR keeps executor skill discovery and reads on the filesystem that
owns the selected root while reusing the existing skill rules.

## What changed

- Generalize the existing skill traversal to operate on `PathUri`
through `ExecutorFileSystem`, preserving its depth, directory, symlink,
and sibling-metadata concurrency behavior.
- Add a small environment skill loader that reuses the shared discovery,
frontmatter validation, dependency parsing, product policy, and
prompt-visibility rules.
- Keep the environment id and entrypoint `PathUri` in the skill catalog,
then route `skills.read` back through the same environment filesystem.
- Preserve the executor's path convention when deriving catalog handles,
including literal backslashes in POSIX filenames.
- Resolve plugin namespaces from nearby manifests through URI-native
filesystem reads.
- Cover foreign Windows roots, executor-owned reads, namespaces,
metadata, policy, and path identity.

```text
selected root (PathUri)
        |
        v
shared discovery over ExecutorFileSystem
        |
        v
environment-bound catalog entry --skills.read--> same ExecutorFileSystem
```

No second filesystem abstraction or duplicate traversal implementation
is introduced.

## Stack

1. #29614 — add lexical `PathUri` containment.
2. #29620 — share URI-native manifest path resolution.
3. #28918 — keep selected plugin roots and resources URI-native.
4. **This PR** — load executor skills without host path conversion.
5. #29628 — resolve executor MCP working directories without host path
conversion.

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

@@ -10,6 +10,7 @@ pub mod plugin_namespace;
 pub use plugin_namespace::DISCOVERABLE_PLUGIN_MANIFEST_PATHS;
 pub use plugin_namespace::find_plugin_manifest_path;
 pub use plugin_namespace::plugin_namespace_for_skill_path;
+pub use plugin_namespace::plugin_namespace_for_skill_uri;
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct PluginSkillRoot {

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

@@ -26,13 +26,12 @@ struct RawPluginManifestName {
 
 async fn plugin_manifest_name(
     fs: &dyn ExecutorFileSystem,
-    plugin_root: &AbsolutePathBuf,
+    plugin_root: &PathUri,
 ) -> Option<String> {
     let mut manifest_path = None;
     for relative_path in DISCOVERABLE_PLUGIN_MANIFEST_PATHS {
-        let candidate = plugin_root.join(relative_path);
-        let candidate_uri = PathUri::from_abs_path(&candidate);
-        match fs.get_metadata(&candidate_uri, /*sandbox*/ None).await {
+        let candidate = plugin_root.join(relative_path).ok()?;
+        match fs.get_metadata(&candidate, /*sandbox*/ None).await {
             Ok(metadata) if metadata.is_file => {
                 manifest_path = Some(candidate);
                 break;
@@ -40,20 +39,16 @@ async fn plugin_manifest_name(
             Ok(_) | Err(_) => {}
         }
     }
-    let manifest_path = manifest_path?;
-    let manifest_path_uri = PathUri::from_abs_path(&manifest_path);
     let contents = fs
-        .read_file_text(&manifest_path_uri, /*sandbox*/ None)
+        .read_file_text(&manifest_path?, /*sandbox*/ None)
         .await
         .ok()?;
     let RawPluginManifestName { name: raw_name } = serde_json::from_str(&contents).ok()?;
     Some(
         plugin_root
-            .file_name()
-            .and_then(|entry| entry.to_str())
+            .basename()
             .filter(|_| raw_name.trim().is_empty())
-            .unwrap_or(raw_name.as_str())
-            .to_string(),
+            .unwrap_or(raw_name),
     )
 }
 
@@ -63,10 +58,20 @@ pub async fn plugin_namespace_for_skill_path(
     fs: &dyn ExecutorFileSystem,
     path: &AbsolutePathBuf,
 ) -> Option<String> {
-    for ancestor in path.ancestors() {
-        if let Some(name) = plugin_manifest_name(fs, &ancestor).await {
+    plugin_namespace_for_skill_uri(fs, &PathUri::from_abs_path(path)).await
+}
+
+/// Returns the plugin manifest `name` for the nearest URI ancestor of `path`.
+pub async fn plugin_namespace_for_skill_uri(
+    fs: &dyn ExecutorFileSystem,
+    path: &PathUri,
+) -> Option<String> {
+    let mut ancestor = Some(path.clone());
+    while let Some(path) = ancestor {
+        if let Some(name) = plugin_manifest_name(fs, &path).await {
             return Some(name);
         }
+        ancestor = path.parent();
     }
     None
 }