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/chatwidget/protocol.rs

@@ -224,6 +224,7 @@ impl ChatWidget {
             | ServerNotification::ThreadStarted(_)
             | ServerNotification::ThreadStatusChanged(_)
             | ServerNotification::ThreadArchived(_)
+            | ServerNotification::ThreadDeleted(_)
             | ServerNotification::ThreadUnarchived(_)
             | ServerNotification::RawResponseItemCompleted(_)
             | ServerNotification::CommandExecOutputDelta(_)