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/src/protocol/common.rs

@@ -480,6 +480,11 @@ client_request_definitions! {
         serialization: thread_id(params.thread_id),
         response: v2::ThreadArchiveResponse,
     },
+    ThreadDelete => "thread/delete" {
+        params: v2::ThreadDeleteParams,
+        serialization: thread_id(params.thread_id),
+        response: v2::ThreadDeleteResponse,
+    },
     ThreadUnsubscribe => "thread/unsubscribe" {
         params: v2::ThreadUnsubscribeParams,
         serialization: thread_id(params.thread_id),
@@ -1534,6 +1539,7 @@ server_notification_definitions! {
     ThreadStarted => "thread/started" (v2::ThreadStartedNotification),
     ThreadStatusChanged => "thread/status/changed" (v2::ThreadStatusChangedNotification),
     ThreadArchived => "thread/archived" (v2::ThreadArchivedNotification),
+    ThreadDeleted => "thread/deleted" (v2::ThreadDeletedNotification),
     ThreadUnarchived => "thread/unarchived" (v2::ThreadUnarchivedNotification),
     ThreadClosed => "thread/closed" (v2::ThreadClosedNotification),
     SkillsChanged => "skills/changed" (v2::SkillsChangedNotification),