[codex] Pass plugin namespace into skill loading (#28608)

## What changed

- retain the parsed plugin manifest namespace on loaded plugins
- carry that namespace through `PluginSkillRoot` and `SkillRoot`
- use the provided namespace when qualifying plugin skill names
- include the namespace in the skills cache key

## Why

Plugin loading has already parsed `plugin.json`, but skill parsing
currently walks every `SKILL.md` ancestor and probes/reads the manifest
again to reconstruct the same namespace. Passing the parsed namespace
removes those repeated filesystem calls, which are particularly costly
on remote filesystems.

Context:
https://openai.slack.com/archives/C0ARA9GF5D4/p1781639496496439?thread_ts=1781202444.891669&cid=C0ARA9GF5D4

## Impact

Plugin skill names remain unchanged. A regression test uses a
deliberately different on-disk manifest name to verify that plugin roots
use the provided parsed namespace.

## Validation

- `just test -p codex-core-skills -p codex-core-plugins -p codex-plugin
-p codex-utils-plugins` (352 passed)
- `just fix -p codex-core-skills -p codex-core-plugins -p codex-plugin
-p codex-utils-plugins`
- `just fmt`

codex-rs/core-plugins/src/manager_tests.rs

@@ -623,6 +623,7 @@ async fn load_plugins_loads_default_skills_and_mcp_servers() {
         vec![LoadedPlugin {
             config_name: "sample@test".to_string(),
             manifest_name: Some("sample".to_string()),
+            plugin_namespace: Some("sample".to_string()),
             manifest_description: Some(
                 "Plugin that includes the sample MCP server and Skills".to_string(),
             ),
@@ -1446,23 +1447,30 @@ async fn load_plugin_skills_dedupes_overlapping_manifest_roots() {
         &plugin_root.join("skills/edk/SKILL.md"),
         "---\nname: edk\ndescription: edk skill\n---\n",
     );
-    let manifest_paths = crate::manifest::PluginManifestPaths {
-        skills: vec![
-            plugin_root.join("skills"),
-            plugin_root.join("skills/abc"),
-            plugin_root.join("skills/edk"),
-            plugin_root.join("skills/abc"),
-        ],
-        mcp_servers: None,
-        apps: None,
-        hooks: None,
+    let manifest = crate::manifest::PluginManifest {
+        name: "sample".to_string(),
+        version: None,
+        description: None,
+        keywords: Vec::new(),
+        paths: crate::manifest::PluginManifestPaths {
+            skills: vec![
+                plugin_root.join("skills"),
+                plugin_root.join("skills/abc"),
+                plugin_root.join("skills/edk"),
+                plugin_root.join("skills/abc"),
+            ],
+            mcp_servers: None,
+            apps: None,
+            hooks: None,
+        },
+        interface: None,
     };
     let plugin_id = PluginId::parse("sample@test").expect("plugin id should parse");
 
     let resolved = load_plugin_skills(
         &plugin_root,
         &plugin_id,
-        &manifest_paths,
+        &manifest,
         /*restriction_product*/ None,
         &SkillConfigRules::default(),
     )
@@ -1677,6 +1685,7 @@ async fn load_plugins_preserves_disabled_plugins_without_effective_contributions
         vec![LoadedPlugin {
             config_name: "sample@test".to_string(),
             manifest_name: None,
+            plugin_namespace: None,
             manifest_description: None,
             root: AbsolutePathBuf::try_from(plugin_root).unwrap(),
             enabled: false,
@@ -1845,6 +1854,12 @@ fn capability_index_filters_inactive_and_zero_capability_plugins() {
     let plugin = |config_name: &str, dir_name: &str, manifest_name: &str| LoadedPlugin {
         config_name: config_name.to_string(),
         manifest_name: Some(manifest_name.to_string()),
+        plugin_namespace: Some(
+            config_name
+                .split_once('@')
+                .map_or(config_name, |(name, _)| name)
+                .to_string(),
+        ),
         manifest_description: None,
         root: AbsolutePathBuf::try_from(codex_home.path().join(dir_name)).unwrap(),
         enabled: true,