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/core-plugins/src/loader.rs

@@ -112,7 +112,6 @@ pub async fn load_plugins_from_layer_stack(
     extra_plugins: HashMap<String, PluginConfig>,
     store: &PluginStore,
     restriction_product: Option<Product>,
-    plugin_hooks_enabled: bool,
 ) -> PluginLoadOutcome<McpServerConfig> {
     let skill_config_rules = skill_config_rules_from_stack(config_layer_stack);
     let mut configured_plugins = configured_plugins_from_stack(config_layer_stack);
@@ -129,7 +128,6 @@ pub async fn load_plugins_from_layer_stack(
             store,
             restriction_product,
             &skill_config_rules,
-            plugin_hooks_enabled,
         )
         .await;
         for name in loaded_plugin.mcp_servers.keys() {
@@ -499,7 +497,6 @@ async fn load_plugin(
     store: &PluginStore,
     restriction_product: Option<Product>,
     skill_config_rules: &SkillConfigRules,
-    plugin_hooks_enabled: bool,
 ) -> LoadedPlugin<McpServerConfig> {
     let plugin_id = PluginId::parse(&config_name);
     let active_plugin_root = plugin_id
@@ -597,16 +594,14 @@ async fn load_plugin(
     }
     loaded_plugin.mcp_servers = mcp_servers;
     loaded_plugin.apps = load_plugin_apps(plugin_root.as_path()).await;
-    if plugin_hooks_enabled {
-        let (hook_sources, hook_load_warnings) = load_plugin_hooks(
-            &plugin_root,
-            &loaded_plugin_id,
-            &store.plugin_data_root(&loaded_plugin_id),
-            manifest_paths,
-        );
-        loaded_plugin.hook_sources = hook_sources;
-        loaded_plugin.hook_load_warnings = hook_load_warnings;
-    }
+    let (hook_sources, hook_load_warnings) = load_plugin_hooks(
+        &plugin_root,
+        &loaded_plugin_id,
+        &store.plugin_data_root(&loaded_plugin_id),
+        manifest_paths,
+    );
+    loaded_plugin.hook_sources = hook_sources;
+    loaded_plugin.hook_load_warnings = hook_load_warnings;
     loaded_plugin
 }