feat: add diff_status_change event to /chats/watch pubsub stream (#22419)

## Summary

Adds a new `diff_status_change` event kind to the `/chats/watch` pubsub
stream so the sidebar can update diff status (PR created, files changed,
branch info) without a full page reload.

### Problem

When a chat's diff status changes (e.g. PR created via GitHub, git
branch pushed), the sidebar didn't update because:
1. The backend `publishChatPubsubEvent` didn't include diff status data
2. The frontend watch handler only merged `status`, `title`, and
`updated_at` from events

### Solution

A **notify-only** approach: a new `ChatEventKindDiffStatusChange` event
kind tells the frontend "diff status changed for chat X" — the frontend
then invalidates the relevant React Query cache entries to re-fetch.

### Backend changes

- **`coderd/pubsub/chatevent.go`**: New `ChatEventKindDiffStatusChange =
"diff_status_change"` constant
- **`coderd/chatd/chatd.go`**: New `PublishDiffStatusChange(ctx,
chatID)` method on `Server`
- **`coderd/chats.go`**: New `publishChatDiffStatusEvent` helper.
Published from:
- `refreshWorkspaceChatDiffStatuses` — after each chat's diff status is
refreshed via GitHub API
- `storeChatGitRef` — after persisting git branch/origin info from
workspace agent

### Frontend changes

- **`AgentsPage.tsx`**: Handle `diff_status_change` event by
invalidating `chatDiffStatusKey` and `chatDiffContentsKey` queries
- **`ChatContext.ts`**: Remove redundant diff status invalidation that
fired on every chat status change (the new event kind handles this
properly)

coderd/pubsub/chatevent.go

@@ -39,8 +39,9 @@ type ChatEvent struct {
 type ChatEventKind string
 
 const (
-	ChatEventKindStatusChange ChatEventKind = "status_change"
-	ChatEventKindTitleChange  ChatEventKind = "title_change"
-	ChatEventKindCreated      ChatEventKind = "created"
-	ChatEventKindDeleted      ChatEventKind = "deleted"
+	ChatEventKindStatusChange     ChatEventKind = "status_change"
+	ChatEventKindTitleChange      ChatEventKind = "title_change"
+	ChatEventKindCreated          ChatEventKind = "created"
+	ChatEventKindDeleted          ChatEventKind = "deleted"
+	ChatEventKindDiffStatusChange ChatEventKind = "diff_status_change"
 )