[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.rs

@@ -1591,7 +1591,7 @@ impl PluginsManager {
         let resolved_skills = load_plugin_skills(
             &source_path,
             &plugin_id,
-            &manifest.paths,
+            &manifest,
             self.restriction_product,
             &codex_core_skills::config_rules::skill_config_rules_from_stack(
                 &config.config_layer_stack,