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/thread-store/src/local/mod.rs

@@ -1,5 +1,6 @@
 mod archive_thread;
 mod create_thread;
+mod delete_thread;
 mod helpers;
 mod list_threads;
 mod live_writer;
@@ -24,6 +25,7 @@ use tokio::sync::Mutex;
 use crate::AppendThreadItemsParams;
 use crate::ArchiveThreadParams;
 use crate::CreateThreadParams;
+use crate::DeleteThreadParams;
 use crate::ListThreadsParams;
 use crate::LoadThreadHistoryParams;
 use crate::ReadThreadByRolloutPathParams;
@@ -288,6 +290,10 @@ impl ThreadStore for LocalThreadStore {
     ) -> ThreadStoreResult<StoredThread> {
         unarchive_thread::unarchive_thread(self, params).await
     }
+
+    async fn delete_thread(&self, params: DeleteThreadParams) -> ThreadStoreResult<()> {
+        delete_thread::delete_thread(self, params).await
+    }
 }
 
 #[cfg(test)]