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/chatd/chatd.go

@@ -1430,6 +1430,24 @@ func (p *Server) publishChatPubsubEvent(chat database.Chat, kind coderdpubsub.Ch
 	}
 }
 
+// PublishDiffStatusChange broadcasts a diff_status_change event for
+// the given chat so that watching clients know to re-fetch the diff
+// status. This is called from the HTTP layer after the diff status
+// is updated in the database.
+func (p *Server) PublishDiffStatusChange(ctx context.Context, chatID uuid.UUID) error {
+	if p.pubsub == nil {
+		return nil
+	}
+
+	chat, err := p.db.GetChatByID(ctx, chatID)
+	if err != nil {
+		return xerrors.Errorf("get chat: %w", err)
+	}
+
+	p.publishChatPubsubEvent(chat, coderdpubsub.ChatEventKindDiffStatusChange)
+	return nil
+}
+
 func (p *Server) publishError(chatID uuid.UUID, message string) {
 	message = strings.TrimSpace(message)
 	if message == "" {