Move the host skills service into the skills extension (#36921)

## What changed

- Move host skill discovery, snapshot caching, and configuration handling from
  `codex-core-skills` to `codex-skills-extension`.
- Rename `SkillsService` and `SkillsLoadInput` to `HostSkillsService` and
  `HostSkillsLoadInput` to distinguish the host implementation from other skill
  providers.
- Keep shared loading primitives and skill outcome modeling in
  `codex-core-skills`.

GitOrigin-RevId: d81a21791d2ef8c066e157e7e538b8cb7ee4c24b

codex-rs/core/src/session/mod.rs

@@ -297,7 +297,7 @@ pub(crate) struct PreviousTurnSettings {
     pub(crate) realtime_active: Option<bool>,
 }
 
-use crate::SkillsService;
+use crate::HostSkillsService;
 use crate::exec_policy::ExecPolicyUpdateError;
 use crate::guardian::GuardianReviewOptions;
 use crate::guardian::GuardianReviewSessionManager;
@@ -422,7 +422,7 @@ pub(crate) struct SessionSpawnArgs {
     pub(crate) auth_manager: Arc<AuthManager>,
     pub(crate) models_manager: SharedModelsManager,
     pub(crate) environment_manager: Arc<EnvironmentManager>,
-    pub(crate) skills_service: Arc<SkillsService>,
+    pub(crate) skills_service: Arc<HostSkillsService>,
     pub(crate) plugins_manager: Arc<PluginsManager>,
     pub(crate) mcp_manager: Arc<McpManager>,
     pub(crate) code_mode_session_provider: Arc<dyn codex_code_mode::CodeModeSessionProvider>,