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/lib.rs

@@ -25,6 +25,7 @@ pub use types::AppendThreadItemsParams;
 pub use types::ArchiveThreadParams;
 pub use types::ClearableField;
 pub use types::CreateThreadParams;
+pub use types::DeleteThreadParams;
 pub use types::ExtraConfig;
 pub use types::GitInfoPatch;
 pub use types::ItemPage;