[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/ext/skills/src/provider/executor.rs

@@ -81,6 +81,7 @@ impl SkillProvider for ExecutorSkillProvider {
                         scope: SkillScope::User,
                         file_system: Arc::clone(&file_system),
                         plugin_id: None,
+                        plugin_namespace: None,
                         plugin_root: None,
                     }])
                     .await,

codex-rs/ext/skills/tests/executor_file_system_authority.rs

@@ -196,6 +196,7 @@ async fn skill_loading_and_reads_use_the_supplied_executor_file_system() {
             canonical_root: canonical_root.clone(),
         }),
         plugin_id: None,
+        plugin_namespace: None,
         plugin_root: None,
     }])
     .await;