Replace SkillsManager with SkillsService (#28705)

## Why

Host skill discovery was still exposed as a manager even though it is a
process-owned service shared by sessions, the app-server catalog, and
file-watcher invalidation. The skills extension also consumed an ad hoc
loaded-skills wrapper instead of a named immutable snapshot.

## What changed

- replace `SkillsManager` with concrete `SkillsService`
- make the service cache and return immutable `HostSkillsSnapshot`
values
- migrate the skills extension host provider to the snapshot boundary
- migrate app-server catalog, watcher, and invalidation paths to the
service

This keeps the service limited to host discovery, caching, roots, and
invalidation. Catalog rendering and invocation remain extension
responsibilities for the next stacked change.

codex-rs/core-api/src/lib.rs

@@ -45,7 +45,7 @@ pub use codex_core::config::ThreadStoreConfig;
 pub use codex_core::config::find_codex_home;
 pub use codex_core::init_state_db;
 pub use codex_core::resolve_installation_id;
-pub use codex_core::skills::SkillsManager;
+pub use codex_core::skills::SkillsService;
 pub use codex_core::thread_store_from_config;
 pub use codex_exec_server::EnvironmentManager;
 pub use codex_exec_server::ExecServerError;