Load plugin hooks without other plugin capabilities (#26272)

## Summary

`hooks/list` only consumes plugin hook declarations, but previously
loaded every enabled plugin's skills, MCP configuration, apps, and
capability summary before discarding them.

In a local benchmark, this reduced `hooks/list` latency by over 100ms
(e.g., from 594 to 467ms on startup, and 168 to 16ms when making a
`hooks/list` call later in the same TUI session). This is on the
critical path to rendering the TUI, so every 10s of ms should be eyed
skeptically (IMO).

This change adds a hook-specific plugin loading path that preserves
plugin enablement, remote/local conflict resolution, deterministic
ordering, manifest resolution, and hook-loading warnings while skipping
unrelated capabilities. (I think there's room for a more general design
here that allows you to project the capabilities you need at load-time,
but that seems unnecessary right now.)

codex-rs/core-plugins/src/manager.rs

@@ -2,11 +2,13 @@ use super::PluginLoadOutcome;
 use super::startup_remote_sync::start_startup_remote_plugin_sync_once;
 use crate::OPENAI_CURATED_MARKETPLACE_NAME;
 use crate::installed_marketplaces::installed_marketplace_roots_from_layer_stack;
+use crate::loader::PluginHookLoadOutcome;
 use crate::loader::configured_curated_plugin_ids_from_codex_home;
 use crate::loader::curated_plugin_cache_version;
 use crate::loader::installed_plugin_telemetry_metadata;
 use crate::loader::load_plugin_apps;
 use crate::loader::load_plugin_hooks;
+use crate::loader::load_plugin_hooks_from_layer_stack;
 use crate::loader::load_plugin_mcp_servers;
 use crate::loader::load_plugin_skills;
 use crate::loader::load_plugins_from_layer_stack;
@@ -543,6 +545,24 @@ impl PluginsManager {
         .await
     }
 
+    /// Resolve plugin hooks for a config layer stack without loading other plugin capabilities.
+    pub async fn plugin_hooks_for_layer_stack(
+        &self,
+        config_layer_stack: &ConfigLayerStack,
+        config: &PluginsConfigInput,
+    ) -> PluginHookLoadOutcome {
+        if !config.plugins_enabled {
+            return PluginHookLoadOutcome::default();
+        }
+        load_plugin_hooks_from_layer_stack(
+            config_layer_stack,
+            self.remote_installed_plugin_configs(),
+            &self.store,
+            config.remote_plugin_enabled,
+        )
+        .await
+    }
+
     /// Resolve plugin skill roots for a config layer stack without touching the plugins cache.
     pub async fn effective_skill_roots_for_layer_stack(
         &self,