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/src/config/mod.rs

@@ -1282,7 +1282,6 @@ impl Config {
             self.config_layer_stack.clone(),
             self.features.enabled(Feature::Plugins),
             self.features.enabled(Feature::RemotePlugin),
-            self.features.enabled(Feature::PluginHooks),
             self.chatgpt_base_url.clone(),
         )
     }