Load host skill roots through the skills extension (#37162)

## What changed

- Load ordinary recursive host skill roots with the skills extension's host loader.
- Keep plugin-specific roots on the existing loader so plugin snapshot caching and namespacing continue to work.
- Merge both kinds of root snapshots through the existing precedence and deduplication logic, including discovery paths and source filesystem mappings.
- Move `HostSkillsSnapshot` into the skills extension as the owner of host skill loading.

## Testing

- Cover merged host and plugin catalogs, precedence for symlinked roots, discovery paths, and end-to-end rendering and invocation of host, repository, and plugin skills.

GitOrigin-RevId: c18080afb917c33c57bc17e36b40dd0f3c72d1d7

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

@@ -6,7 +6,7 @@ mod executor;
 mod host;
 mod orchestrator;
 
-use codex_core_skills::HostSkillsSnapshot;
+use crate::HostSkillsSnapshot;
 use codex_exec_server::ExecutorCapabilityDiscoverySnapshot;
 use codex_exec_server::FileSystemSandboxContext;
 use codex_exec_server::ResolvedSelectedCapabilityRoot;