Remove plugin hooks feature flag (#22552)

# Why

This is a follow-up stacked on top of the `plugin_hooks` default-on
change. Once we are comfortable making plugin hooks part of the normal
plugin behavior, the separate feature flag stops buying us much and
leaves extra branching/cache state behind.

# What

- remove the `PluginHooks` feature and generated config-schema entries
- make plugin hook loading/listing follow plugin enablement directly
- drop plugin-manager cache/state that only existed to distinguish
hook-flag toggles
- remove tests and fixtures that modeled `plugin_hooks = true/false`

codex-rs/features/src/tests.rs

@@ -76,6 +76,13 @@ fn apply_patch_freeform_is_removed_and_disabled_by_default() {
     );
 }
 
+#[test]
+fn plugin_hooks_is_removed_and_disabled_by_default() {
+    assert_eq!(Feature::PluginHooks.stage(), Stage::Removed);
+    assert_eq!(Feature::PluginHooks.default_enabled(), false);
+    assert_eq!(feature_for_key("plugin_hooks"), Some(Feature::PluginHooks));
+}
+
 #[test]
 fn code_mode_only_requires_code_mode() {
     let mut features = Features::with_defaults();
@@ -194,12 +201,6 @@ fn tool_search_is_removed_and_disabled_by_default() {
     assert_eq!(feature_for_key("tool_search"), Some(Feature::ToolSearch));
 }
 
-#[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);
@@ -499,6 +500,22 @@ fn from_sources_ignores_removed_apply_patch_freeform_feature_key() {
     assert_eq!(features, Features::with_defaults());
 }
 
+#[test]
+fn from_sources_ignores_removed_plugin_hooks_feature_key() {
+    let features_toml = FeaturesToml::from(BTreeMap::from([("plugin_hooks".to_string(), true)]));
+
+    let features = Features::from_sources(
+        FeatureConfigSource {
+            features: Some(&features_toml),
+            ..Default::default()
+        },
+        FeatureConfigSource::default(),
+        FeatureOverrides::default(),
+    );
+
+    assert_eq!(features, Features::with_defaults());
+}
+
 #[test]
 fn multi_agent_v2_feature_config_deserializes_boolean_toggle() {
     let features: FeaturesToml = toml::from_str(