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/app-server/src/request_processors/catalog_processor.rs

@@ -612,15 +612,10 @@ impl CatalogRequestProcessor {
                 .await;
             let plugins_enabled =
                 config.features.enabled(Feature::Plugins) && workspace_codex_plugins_enabled;
-            let plugin_outcome = if plugins_enabled && config.features.enabled(Feature::PluginHooks)
-            {
+            let plugin_outcome = if plugins_enabled {
                 let plugins_input = config.plugins_config_input();
                 plugins_manager
-                    .plugins_for_layer_stack(
-                        &config.config_layer_stack,
-                        &plugins_input,
-                        /*plugin_hooks_feature_enabled*/ true,
-                    )
+                    .plugins_for_layer_stack(&config.config_layer_stack, &plugins_input)
                     .await
             } else {
                 PluginLoadOutcome::default()