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/tui/src/app/app_server_event_targets.rs

@@ -49,6 +49,7 @@ pub(super) fn server_notification_thread_target(
             Some(notification.thread_id.as_str())
         }
         ServerNotification::ThreadArchived(notification) => Some(notification.thread_id.as_str()),
+        ServerNotification::ThreadDeleted(notification) => Some(notification.thread_id.as_str()),
         ServerNotification::ThreadUnarchived(notification) => Some(notification.thread_id.as_str()),
         ServerNotification::ThreadClosed(notification) => Some(notification.thread_id.as_str()),
         ServerNotification::ThreadNameUpdated(notification) => {