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/core/src/session/turn_context.rs

@@ -3,7 +3,6 @@ use crate::environment_selection::TurnEnvironmentSnapshot;
 use crate::shell_snapshot::ShellSnapshotFile;
 use codex_core_plugins::PluginCommandAttribution;
 use codex_core_plugins::TrustedPluginRoots;
-use codex_core_skills::HostSkillsSnapshot;
 use codex_file_system::FileSystemSandboxContext;
 use codex_model_provider::SharedModelProvider;
 use codex_protocol::SessionId;
@@ -15,6 +14,7 @@ use codex_protocol::protocol::MultiAgentVersion;
 use codex_protocol::protocol::ThreadHistoryMode;
 use codex_protocol::protocol::TurnEnvironmentSelection;
 use codex_sandboxing::policy_transforms::effective_permission_profile;
+use codex_skills_extension::HostSkillsSnapshot;
 use codex_utils_path_uri::PathUri;
 use futures::FutureExt;
 use futures::future::BoxFuture;