feat: agents git watch backend (#22565)

Adds real-time git status watching for workspace agents, so the frontend
can subscribe over WebSocket and show
git file changes in near real-time.

1. Subscription is scoped to a **chat** via `GET
/api/experimental/chats/{chat}/git/watch`.
2. The workspace agent automatically determines which paths to watch
based on tool calls made by the chat (and its ancestor chats).
3. Workspace agent polls subscribed repo working trees on a 30s
interval, on tools calls, and on explicit `refresh` from the client.
4. Scans are rate-limited to at most once per second.
5. Edited paths are tracked **in-memory** inside the workspace agent.
There is no database persistence — state is lost on agent restart. This
will be addresses in a future PR.
6. Messages sent over WebSocket include a full-repo snapshot (unified
diff, branch, origin). A new message is emitted only when the snapshot
changes.

This PR was implemented with AI with me closely controlling what it's
doing. The code follows a plan file that was updated continuously during
implementation. Here's the file if you'd like to see it:
[project.md](https://gist.github.com/hugodutka/8722cf80c92f8a56555f7bc595b770e2).
It reflects the current state of the PR.

docs/reference/api/chats.md

@@ -18,6 +18,32 @@ curl -X POST http://coder-server:8080/api/v2/chats/{chat}/archive
 |--------|-----------------------------------------------------------------|-------------|--------|
 | 204    | [No Content](https://tools.ietf.org/html/rfc7231#section-6.3.5) | No Content  |        |
 
+## Watch git changes for a chat
+
+### Code samples
+
+```shell
+# Example request using curl
+curl -X GET http://coder-server:8080/api/v2/chats/{chat}/git/watch \
+  -H 'Coder-Session-Token: API_KEY'
+```
+
+`GET /chats/{chat}/git/watch`
+
+### Parameters
+
+| Name   | In   | Type         | Required | Description |
+|--------|------|--------------|----------|-------------|
+| `chat` | path | string(uuid) | true     | Chat ID     |
+
+### Responses
+
+| Status | Meaning                                                                  | Description         | Schema |
+|--------|--------------------------------------------------------------------------|---------------------|--------|
+| 101    | [Switching Protocols](https://tools.ietf.org/html/rfc7231#section-6.2.2) | Switching Protocols |        |
+
+To perform this operation, you must be authenticated. [Learn more](authentication.md).
+
 ## Unarchive a chat
 
 ### Code samples