core: Make FileWatcher reusable (#15093)

### Summary
Make `FileWatcher` a reusable core component which can be built upon.
Extract skills-related logic into a separate `SkillWatcher`.
Introduce a composable `ThrottledWatchReceiver` to throttle filesystem
events, coalescing affected paths among them.

### Testing
Updated existing unit tests.

codex-rs/core/src/codex_tests_guardian.rs

@@ -435,7 +435,7 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
         true,
     ));
     let mcp_manager = Arc::new(McpManager::new(Arc::clone(&plugins_manager)));
-    let file_watcher = Arc::new(FileWatcher::noop());
+    let skills_watcher = Arc::new(SkillsWatcher::noop());
 
     let CodexSpawnOk { codex, .. } = Codex::spawn(CodexSpawnArgs {
         config,
@@ -444,7 +444,7 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
         skills_manager,
         plugins_manager,
         mcp_manager,
-        file_watcher,
+        skills_watcher,
         conversation_history: InitialHistory::New,
         session_source: SessionSource::SubAgent(SubAgentSource::Other(
             GUARDIAN_REVIEWER_NAME.to_string(),