[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/plugin/src/load_outcome.rs

@@ -17,6 +17,7 @@ const MAX_CAPABILITY_SUMMARY_DESCRIPTION_LEN: usize = 1024;
 pub struct LoadedPlugin<M> {
     pub config_name: String,
     pub manifest_name: Option<String>,
+    pub plugin_namespace: Option<String>,
     pub manifest_description: Option<String>,
     pub root: AbsolutePathBuf,
     pub enabled: bool,
@@ -126,11 +127,15 @@ impl<M: Clone> PluginLoadOutcome<M> {
         let mut skill_roots = Vec::new();
         let mut seen_paths = HashSet::new();
         for plugin in self.plugins.iter().filter(|plugin| plugin.is_active()) {
+            let Some(plugin_namespace) = &plugin.plugin_namespace else {
+                continue;
+            };
             for path in &plugin.skill_roots {
                 if seen_paths.insert(path.clone()) {
                     skill_roots.push(PluginSkillRoot {
                         path: path.clone(),
                         plugin_id: plugin.config_name.clone(),
+                        plugin_namespace: plugin_namespace.clone(),
                         plugin_root: plugin.root.clone(),
                     });
                 }
@@ -218,6 +223,12 @@ mod tests {
         LoadedPlugin {
             config_name: config_name.to_string(),
             manifest_name: None,
+            plugin_namespace: Some(
+                config_name
+                    .split_once('@')
+                    .map_or(config_name, |(name, _)| name)
+                    .to_string(),
+            ),
             manifest_description: None,
             root: test_path(config_name),
             enabled: true,
@@ -245,6 +256,7 @@ mod tests {
             vec![PluginSkillRoot {
                 path: shared_root,
                 plugin_id: "zeta@test".to_string(),
+                plugin_namespace: "zeta".to_string(),
                 plugin_root: test_path("zeta@test"),
             }]
         );