Support Agent Plugins manifests (#35105)

## What changed

- Recognize root `plugin.json` files using the Agent Plugins 1.0 schema and map their portable metadata, `skills/`, and `mcp.json` into Codex plugin manifests.
- Apply Codex-specific apps, hooks, and interface settings from the inline `com.openai` extension, with `.codex-plugin/plugin.json` as a fallback overlay.
- Preserve legacy manifest precedence when a root `plugin.json` is unrelated, and reject unsupported Agent Plugins schema versions.
- Add a direct-child skill discovery mode that excludes nested skills and paths resolving outside the plugin root.

## Testing

- Cover manifest metadata, validation, extension precedence, legacy fallback, and direct-child skill path boundaries.

GitOrigin-RevId: eab24139f13a5cc5cb3ad3fb444d8e904511aca6

codex-rs/ext/skills/src/provider/host_tests.rs

@@ -38,6 +38,7 @@ async fn host_catalog_entries_carry_their_prompt_scope() -> Result<(), Box<dyn s
             plugin_id: None,
             plugin_namespace: None,
             plugin_root: None,
+            discovery_mode: Default::default(),
         }],
         /*plugin_skill_snapshots*/ None,
         Arc::new(Semaphore::new(1)),