Add app-server thread/delete API (#25018)

## Why

Clients can archive and unarchive threads today, but there is no
app-server API for permanently removing a thread. Deletion also needs to
cover the full session tree: deleting a main thread should remove
spawned subagent threads and the related local metadata instead of
leaving orphaned rollout files, goals, or subagent state behind.

## What

- Adds the v2 `thread/delete` request and `thread/deleted` notification,
with the response shape kept consistent with `thread/archive`.
- Implements local hard delete for active and archived rollout files.
- Deletes the requested thread's state DB row as the commit point, then
best-effort cleans associated state including spawned descendants,
goals, spawn edges, logs, dynamic tools, and agent job assignments.
- Updates app-server API docs and generated protocol schema/TypeScript
fixtures.

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

@@ -3523,6 +3523,17 @@
       ],
       "type": "object"
     },
+    "ThreadDeletedNotification": {
+      "properties": {
+        "threadId": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "threadId"
+      ],
+      "type": "object"
+    },
     "ThreadGoal": {
       "properties": {
         "createdAt": {
@@ -5447,6 +5458,26 @@
       "title": "Thread/archivedNotification",
       "type": "object"
     },
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "thread/deleted"
+          ],
+          "title": "Thread/deletedNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/ThreadDeletedNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "Thread/deletedNotification",
+      "type": "object"
+    },
     {
       "properties": {
         "method": {