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/core-plugins/src/loader.rs

@@ -45,6 +45,7 @@ use codex_protocol::protocol::SkillScope;
 use codex_skills::SkillConfigRules;
 use codex_skills::SkillMetadata;
 use codex_utils_absolute_path::AbsolutePathBuf;
+use codex_utils_plugins::SkillDiscoveryMode;
 use codex_utils_plugins::find_plugin_manifest_path;
 use serde_json::Value as JsonValue;
 use std::collections::HashMap;
@@ -955,6 +956,7 @@ pub(crate) async fn load_plugin_skill_inventory(
             plugin_id: Some(plugin_id.as_key()),
             plugin_namespace: Some(manifest.name.clone()),
             plugin_root: Some(plugin_root.clone()),
+            discovery_mode: SkillDiscoveryMode::Recursive,
         })
         .collect::<Vec<_>>();
     let outcome = load_skills_from_roots(roots, plugin_skill_snapshots, root_scan_slots).await;