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.

coderd/apidoc/swagger.json

@@ -422,6 +422,33 @@
 				}
 			}
 		},
+		"/chats/{chat}/git/watch": {
+			"get": {
+				"security": [
+					{
+						"CoderSessionToken": []
+					}
+				],
+				"tags": ["Chats"],
+				"summary": "Watch git changes for a chat.",
+				"operationId": "watch-chat-git",
+				"parameters": [
+					{
+						"type": "string",
+						"format": "uuid",
+						"description": "Chat ID",
+						"name": "chat",
+						"in": "path",
+						"required": true
+					}
+				],
+				"responses": {
+					"101": {
+						"description": "Switching Protocols"
+					}
+				}
+			}
+		},
 		"/chats/{chat}/unarchive": {
 			"post": {
 				"tags": ["Chats"],