Enable plugin hooks by default (#22549)

# Why

Plugin-bundled hooks are already wired through the plugin manager,
session setup, and app-server hook listing paths. Keeping `plugin_hooks`
disabled by default means users still need an explicit feature opt-in
before that existing behavior participates in normal plugin loading.

# What

- mark `plugin_hooks` as stable and enable it by default
- add feature-registry test coverage for the new default/stage pairing

Validation:

- `cargo test -p codex-features`
- `just fmt`

codex-rs/features/src/tests.rs

@@ -183,6 +183,12 @@ fn tool_search_is_stable_and_enabled_by_default() {
     assert_eq!(Feature::ToolSearch.default_enabled(), true);
 }
 
+#[test]
+fn plugin_hooks_are_stable_and_enabled_by_default() {
+    assert_eq!(Feature::PluginHooks.stage(), Stage::Stable);
+    assert_eq!(Feature::PluginHooks.default_enabled(), true);
+}
+
 #[test]
 fn browser_controls_are_stable_and_enabled_by_default() {
     assert_eq!(Feature::InAppBrowser.stage(), Stage::Stable);