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/plugin/src/lib.rs

@@ -6,6 +6,8 @@ pub use codex_utils_plugins::plugin_namespace_for_skill_path;
 mod load_outcome;
 mod plugin_id;
 
+use codex_config::HookEventsToml;
+use codex_utils_absolute_path::AbsolutePathBuf;
 pub use load_outcome::EffectiveSkillRoots;
 pub use load_outcome::LoadedPlugin;
 pub use load_outcome::PluginLoadOutcome;
@@ -27,6 +29,16 @@ pub struct PluginCapabilitySummary {
     pub app_connector_ids: Vec<AppConnectorId>,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PluginHookSource {
+    pub plugin_id: PluginId,
+    pub plugin_root: AbsolutePathBuf,
+    pub plugin_data_root: AbsolutePathBuf,
+    pub source_path: AbsolutePathBuf,
+    pub source_relative_path: String,
+    pub hooks: HookEventsToml,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct PluginTelemetryMetadata {
     pub plugin_id: PluginId,

codex-rs/plugin/src/load_outcome.rs

@@ -5,6 +5,7 @@ use codex_utils_absolute_path::AbsolutePathBuf;
 
 use crate::AppConnectorId;
 use crate::PluginCapabilitySummary;
+use crate::PluginHookSource;
 
 const MAX_CAPABILITY_SUMMARY_DESCRIPTION_LEN: usize = 1024;
 
@@ -21,6 +22,8 @@ pub struct LoadedPlugin<M> {
     pub has_enabled_skills: bool,
     pub mcp_servers: HashMap<String, M>,
     pub apps: Vec<AppConnectorId>,
+    pub hook_sources: Vec<PluginHookSource>,
+    pub hook_load_warnings: Vec<String>,
     pub error: Option<String>,
 }
 
@@ -140,6 +143,22 @@ impl<M: Clone> PluginLoadOutcome<M> {
         apps
     }
 
+    pub fn effective_plugin_hook_sources(&self) -> Vec<PluginHookSource> {
+        self.plugins
+            .iter()
+            .filter(|plugin| plugin.is_active())
+            .flat_map(|plugin| plugin.hook_sources.iter().cloned())
+            .collect()
+    }
+
+    pub fn effective_plugin_hook_warnings(&self) -> Vec<String> {
+        self.plugins
+            .iter()
+            .filter(|plugin| plugin.is_active())
+            .flat_map(|plugin| plugin.hook_load_warnings.iter().cloned())
+            .collect()
+    }
+
     pub fn capability_summaries(&self) -> &[PluginCapabilitySummary] {
         &self.capability_summaries
     }