Show plugin hooks in plugin details (#21447)

Supersedes the abandoned #19859, rebuilt on latest `main`.

# Why

PR #19705 adds discovery for hooks bundled with plugins, but `/plugins`
still only shows skills, apps, and MCP servers. This follow-up makes
bundled hooks visible in the same plugin detail view so users can
inspect the full plugin surface in one place.

We also need `PluginHookSummary` to populate Plugin Hooks in the app;
`hooks/list` is not enough there because plugin detail needs to show
hooks for disabled plugins too.

# What

- extend `plugin/read` with `PluginHookSummary` entries for bundled
hooks
- summarize plugin hooks while loading plugin details
- render a `Hooks` row in the `/plugins` detail popup

<img width="3456" height="848" alt="CleanShot 2026-04-27 at 11 45 34@2x"
src="https://github.com/user-attachments/assets/fe3a38d6-a260-4351-8513-fb04c93d725b"
/>

codex-rs/hooks/src/config_rules.rs

@@ -13,7 +13,7 @@ use codex_config::TomlValue;
 /// disabled layers, to match the skills config behavior. Project, managed, and
 /// plugin layers can discover hooks, but they do not get to write user hook
 /// state.
-pub(crate) fn hook_states_from_stack(
+pub fn hook_states_from_stack(
     config_layer_stack: Option<&ConfigLayerStack>,
 ) -> HashMap<String, HookStateToml> {
     let Some(config_layer_stack) = config_layer_stack else {