Load plugin hooks without other plugin capabilities (#26272)

## Summary

`hooks/list` only consumes plugin hook declarations, but previously
loaded every enabled plugin's skills, MCP configuration, apps, and
capability summary before discarding them.

In a local benchmark, this reduced `hooks/list` latency by over 100ms
(e.g., from 594 to 467ms on startup, and 168 to 16ms when making a
`hooks/list` call later in the same TUI session). This is on the
critical path to rendering the TUI, so every 10s of ms should be eyed
skeptically (IMO).

This change adds a hook-specific plugin loading path that preserves
plugin enablement, remote/local conflict resolution, deterministic
ordering, manifest resolution, and hook-loading warnings while skipping
unrelated capabilities. (I think there's room for a more general design
here that allows you to project the capabilities you need at load-time,
but that seems unnecessary right now.)

codex-rs/core-plugins/src/manager_tests.rs

@@ -3858,3 +3858,50 @@ async fn load_plugins_ignores_project_config_files() {
 
     assert_eq!(outcome, PluginLoadOutcome::default());
 }
+
+#[tokio::test]
+async fn plugin_hooks_for_layer_stack_loads_configured_plugin_hooks() {
+    let codex_home = TempDir::new().unwrap();
+    let plugin_root = codex_home
+        .path()
+        .join("plugins/cache")
+        .join("test/sample/local");
+    write_plugin(
+        codex_home.path().join("plugins/cache/test").as_path(),
+        "sample/local",
+        "sample",
+    );
+    write_file(
+        &plugin_root.join("hooks/hooks.json"),
+        r#"{
+  "hooks": {
+    "SessionStart": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "echo startup"
+          }
+        ]
+      }
+    ]
+  }
+}"#,
+    );
+    write_file(
+        &codex_home.path().join(CONFIG_TOML_FILE),
+        &plugin_config_toml(/*enabled*/ true, /*plugins_feature_enabled*/ true),
+    );
+    let config = load_config(codex_home.path(), codex_home.path()).await;
+
+    let outcome = PluginsManager::new(codex_home.path().to_path_buf())
+        .plugin_hooks_for_layer_stack(&config.config_layer_stack, &config)
+        .await;
+
+    assert_eq!(outcome.hook_sources.len(), 1);
+    assert_eq!(
+        outcome.hook_sources[0].source_relative_path,
+        "hooks/hooks.json"
+    );
+    assert_eq!(outcome.hook_load_warnings, Vec::<String>::new());
+}