Discover hooks bundled with plugins (#19705)

## Why

Plugins can bundle lifecycle hooks, but Codex previously only discovered
hooks from user, project, and managed config layers. This adds the
plugin discovery and runtime plumbing needed for plugin-bundled hooks
while keeping execution behind the `plugin_hooks` feature flag.

## What

- Discovers plugin hook sources from each plugin's default
`hooks/hooks.json`.
- Supports `plugin.json` manifest `hooks` entries as either relative
paths or inline hook objects.
- Plumbs discovered plugin hook sources through plugin loading into the
hook runtime when `plugin_hooks` is enabled.
- Marks plugin-originated hook runs as `HookSource::Plugin`.
- Injects `PLUGIN_ROOT` and `CLAUDE_PLUGIN_ROOT` into plugin hook
command environments.
- Updates generated schemas and hook source metadata for the plugin hook
source.

## Stack

1. This PR - openai/codex#19705
2. openai/codex#19778
3. openai/codex#19840
4. openai/codex#19882

## Reviewer Notes

- Core logic is in `codex-rs/core-plugins/src/loader.rs` and
`codex-rs/hooks/src/engine/discovery.rs`
- Moved existing / adding new tests to
`codex-rs/core-plugins/src/loader_tests.rs` hence the large diff there
- Otherwise mostly plumbing and minor schema updates

### Core Changes

The `codex-rs/core` changes are limited to wiring plugin hook support
into existing core flows:

- `core/src/session/session.rs` conditionally pulls effective plugin
hook sources and plugin hook load warnings from `PluginsManager` when
`plugin_hooks` is enabled, then passes them into `HooksConfig`.
- `core/src/hook_runtime.rs` adds the `plugin` metric tag for
`HookSource::Plugin`.
- `core/config.schema.json` picks up the new `plugin_hooks` feature
flag, and `core/src/plugins/manager_tests.rs` updates fixtures for the
added plugin hook fields.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/features/src/lib.rs

@@ -159,6 +159,8 @@ pub enum Feature {
     ToolSuggest,
     /// Enable plugins.
     Plugins,
+    /// Enable plugin-bundled lifecycle hooks.
+    PluginHooks,
     /// Allow the in-app browser pane in desktop apps.
     ///
     /// Requirements-only gate: this should be set from requirements, not user config.
@@ -876,6 +878,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::PluginHooks,
+        key: "plugin_hooks",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::InAppBrowser,
         key: "in_app_browser",