Add hooks/list app-server RPC (#19778)

## Why

We need a way to list the available hooks to expose via the TUI and App
so users can view and manage their hooks

## What

- Adds `hooks/list` for one or more `cwd` values that returns discovered
hook metadata

## Stack

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

## Review Notes

The generated schema files account for most of the raw diff, these files
have the core change:

- `hooks/src/engine/discovery.rs` builds the inventory entries during
hook discovery while leaving runtime handlers focused on execution.
- `app-server/src/codex_message_processor.rs` wires `hooks/list` into
the app-server flow for each requested `cwd`.
- `app-server-protocol/src/protocol/v2.rs` defines the new v2
request/response payloads exposed on the wire.

### Core Changes

`core/src/plugins/manager.rs` adds `plugins_for_layer_stack(...)` so
`skills/list` and `hooks/list`can resolve plugin state for each
requested `cwd`

---------

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

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -1415,6 +1415,18 @@
       ],
       "type": "object"
     },
+    "HooksListParams": {
+      "properties": {
+        "cwds": {
+          "description": "When empty, defaults to the current session working directory.",
+          "items": {
+            "type": "string"
+          },
+          "type": "array"
+        }
+      },
+      "type": "object"
+    },
     "ImageDetail": {
       "enum": [
         "auto",
@@ -4860,6 +4872,30 @@
       "title": "Skills/listRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "hooks/list"
+          ],
+          "title": "Hooks/listRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/HooksListParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Hooks/listRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {