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/in_memory.rs

@@ -18,6 +18,7 @@ use codex_protocol::protocol::ThreadMemoryMode;
 use crate::AppendThreadItemsParams;
 use crate::ArchiveThreadParams;
 use crate::CreateThreadParams;
+use crate::DeleteThreadParams;
 use crate::ListThreadsParams;
 use crate::LoadThreadHistoryParams;
 use crate::ReadThreadByRolloutPathParams;
@@ -115,6 +116,7 @@ pub struct InMemoryThreadStoreCalls {
     pub update_thread_metadata: usize,
     pub archive_thread: usize,
     pub unarchive_thread: usize,
+    pub delete_thread: usize,
 }
 
 /// In-memory [`ThreadStore`] implementation for tests and debug configs.
@@ -333,6 +335,25 @@ impl ThreadStore for InMemoryThreadStore {
         state.calls.unarchive_thread += 1;
         stored_thread_from_state(&state, params.thread_id, /*include_history*/ false)
     }
+
+    async fn delete_thread(&self, params: DeleteThreadParams) -> ThreadStoreResult<()> {
+        let mut state = self.state.lock().await;
+        state.calls.delete_thread += 1;
+        let existed = state.histories.remove(&params.thread_id).is_some();
+        state.created_threads.remove(&params.thread_id);
+        state.names.remove(&params.thread_id);
+        state.metadata_updates.remove(&params.thread_id);
+        state
+            .rollout_paths
+            .retain(|_, thread_id| *thread_id != params.thread_id);
+        if existed {
+            Ok(())
+        } else {
+            Err(ThreadStoreError::ThreadNotFound {
+                thread_id: params.thread_id,
+            })
+        }
+    }
 }
 
 fn stored_thread_from_state(