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/chats.go

@@ -1068,7 +1068,9 @@ func (api *API) storeChatGitRef(ctx context.Context, workspaceID, workspaceOwner
 				slog.F("workspace_id", workspaceID),
 				slog.Error(err),
 			)
+			continue
 		}
+		api.publishChatDiffStatusEvent(ctx, chat.ID)
 	}
 }
 
@@ -1109,6 +1111,7 @@ func (api *API) refreshWorkspaceChatDiffStatuses(ctx context.Context, workspaceI
 		}
 
 		api.publishChatStatusEvent(ctx, chat.ID)
+		api.publishChatDiffStatusEvent(ctx, chat.ID)
 	}
 
 	return allHavePR
@@ -1138,6 +1141,19 @@ func (api *API) publishChatStatusEvent(ctx context.Context, chatID uuid.UUID) {
 	}
 }
 
+func (api *API) publishChatDiffStatusEvent(ctx context.Context, chatID uuid.UUID) {
+	if api.chatDaemon == nil {
+		return
+	}
+
+	if err := api.chatDaemon.PublishDiffStatusChange(ctx, chatID); err != nil {
+		api.Logger.Debug(ctx, "failed to publish chat diff status change",
+			slog.F("chat_id", chatID),
+			slog.Error(err),
+		)
+	}
+}
+
 func (api *API) resolveChatDiffContents(
 	ctx context.Context,
 	chat database.Chat,