feat(app-server): thread/unsubscribe API (#10954)

Adds a new v2 app-server API for a client to be able to unsubscribe to a
thread:
- New RPC method: `thread/unsubscribe`
- New server notification: `thread/closed`

Today clients can start/resume/archive threads, but there wasn’t a way
to explicitly unload a live thread from memory without archiving it.
With `thread/unsubscribe`, a client can indicate it is no longer
actively working with a live Thread. If this is the only client
subscribed to that given thread, the thread will be automatically closed
by app-server, at which point the server will send `thread/closed` and
`thread/status/changed` with `status: notLoaded` notifications.

This gives clients a way to prevent long-running app-server processes
from accumulating too many thread (and related) objects in memory.

Closed threads will also be removed from `thread/loaded/list`.

codex-rs/app-server/README.md

@@ -127,6 +127,7 @@ Example with notification opt-out:
 - `thread/read` — read a stored thread by id without resuming it; optionally include turns via `includeTurns`. The returned `thread` includes `status` (`ThreadStatus`), defaulting to `notLoaded` when the thread is not currently loaded.
 - `thread/status/changed` — notification emitted when a loaded thread’s status changes (`threadId` + new `status`).
 - `thread/archive` — move a thread’s rollout file into the archived directory; returns `{}` on success and emits `thread/archived`.
+- `thread/unsubscribe` — unsubscribe this connection from thread turn/item events. If this was the last subscriber, the server shuts down and unloads the thread, then emits `thread/closed`.
 - `thread/name/set` — set or update a thread’s user-facing name; returns `{}` on success. Thread names are not required to be unique; name lookups resolve to the most recently updated thread.
 - `thread/unarchive` — move an archived rollout file back into the sessions directory; returns the restored `thread` on success and emits `thread/unarchived`.
 - `thread/compact/start` — trigger conversation history compaction for a thread; returns `{}` immediately while progress streams through standard turn/item notifications.
@@ -283,6 +284,26 @@ When `nextCursor` is `null`, you’ve reached the final page.
 } }
 ```
 
+### Example: Unsubscribe from a loaded thread
+
+`thread/unsubscribe` removes the current connection's subscription to a thread. The response status is one of:
+
+- `unsubscribed` when the connection was subscribed and is now removed.
+- `notSubscribed` when the connection was not subscribed to that thread.
+- `notLoaded` when the thread is not loaded.
+
+If this was the last subscriber, the server unloads the thread and emits `thread/closed` and a `thread/status/changed` transition to `notLoaded`.
+
+```json
+{ "method": "thread/unsubscribe", "id": 22, "params": { "threadId": "thr_123" } }
+{ "id": 22, "result": { "status": "unsubscribed" } }
+{ "method": "thread/status/changed", "params": {
+    "threadId": "thr_123",
+    "status": { "type": "notLoaded" }
+} }
+{ "method": "thread/closed", "params": { "threadId": "thr_123" } }
+```
+
 ### Example: Read a thread
 
 Use `thread/read` to fetch a stored thread by id without resuming it. Pass `includeTurns` when you want the rollout history loaded into `thread.turns`. The returned thread includes `agentNickname` and `agentRole` for AgentControl-spawned thread sub-agents when available.
@@ -555,7 +576,7 @@ Notes:
 
 ## Events
 
-Event notifications are the server-initiated event stream for thread lifecycles, turn lifecycles, and the items within them. After you start or resume a thread, keep reading stdout for `thread/started`, `thread/archived`, `thread/unarchived`, `turn/*`, and `item/*` notifications.
+Event notifications are the server-initiated event stream for thread lifecycles, turn lifecycles, and the items within them. After you start or resume a thread, keep reading stdout for `thread/started`, `thread/archived`, `thread/unarchived`, `thread/closed`, `turn/*`, and `item/*` notifications.
 
 Thread realtime uses a separate thread-scoped notification surface. `thread/realtime/*` notifications are ephemeral transport events, not `ThreadItem`s, and are not returned by `thread/read`, `thread/resume`, or `thread/fork`.