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/tests/suite/hooks.rs

@@ -2934,10 +2934,6 @@ print(json.dumps({{
                 .enable(Feature::Plugins)
                 .expect("test config should allow feature update");
             trust_plugin_hooks(config, plugin_hook_sources);
-            config
-                .features
-                .enable(Feature::PluginHooks)
-                .expect("test config should allow feature update");
         });
     let test = builder.build(&server).await?;