Move direct executor skill discovery into the skills extension (#36880)

## What changed

- Add extension-owned discovery and namespace resolution for skills loaded directly through an `ExecutorFileSystem`.
- Preserve hidden and symlinked skills, nested plugin namespaces, optional `agents/openai.yaml` metadata, product restrictions, and deterministic ordering.
- Reuse the filesystem walk inventory and bound concurrent skill, metadata, and manifest reads.
- Route direct executor catalog loading through the new extension loader.

## Testing

- Cover namespace lookup, metadata probing, walk reuse, concurrent reads, and parity with the existing environment loader.

GitOrigin-RevId: 4e0b821eb84d03f0dc1c2dee7b2b9a072ee3fd44

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

@@ -1,6 +1,5 @@
 use std::sync::Arc;
 
-use codex_core_skills::loader::load_environment_skills_from_root;
 use codex_exec_server::EnvironmentManager;
 use codex_exec_server::FileSystemSandboxContext;
 use codex_protocol::capabilities::CapabilityRootLocation;
@@ -20,6 +19,7 @@ use crate::catalog::SkillResourceId;
 use crate::catalog::SkillSearchResult;
 use crate::catalog::SkillSourceKind;
 use crate::loader::load_environment_skills_from_discovery;
+use crate::loader::load_environment_skills_from_root;
 use crate::provider::MAX_SKILL_RESOURCE_CONTENT_BYTES;
 use crate::provider::SkillListQuery;
 use crate::provider::SkillProvider;