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 == "" {

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,

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"
 )

site/src/pages/AgentsPage/AgentDetail/ChatContext.ts

@@ -1,10 +1,5 @@
 import { watchChat } from "api/api";
-import {
-	chatDiffContentsKey,
-	chatDiffStatusKey,
-	chatKey,
-	chatsKey,
-} from "api/queries/chats";
+import { chatKey, chatsKey } from "api/queries/chats";
 import type * as TypesGen from "api/typesGenerated";
 import { asRecord, asString } from "components/ai-elements/runtimeTypeUtils";
 import {
@@ -614,7 +609,6 @@ export const useChatStore = (
 							continue;
 						}
 
-						const previousStatus = store.getSnapshot().chatStatus;
 						store.setChatStatus(nextStatus);
 						if (nextStatus === "pending" || nextStatus === "waiting") {
 							store.clearStreamState();
@@ -627,16 +621,6 @@ export const useChatStore = (
 							status: nextStatus,
 							updated_at: new Date().toISOString(),
 						}));
-						if (previousStatus !== nextStatus) {
-							void Promise.all([
-								queryClient.invalidateQueries({
-									queryKey: chatDiffStatusKey(chatID),
-								}),
-								queryClient.invalidateQueries({
-									queryKey: chatDiffContentsKey(chatID),
-								}),
-							]);
-						}
 
 						continue;
 					}
@@ -681,7 +665,6 @@ export const useChatStore = (
 		cancelScheduledStreamReset,
 		chatID,
 		clearChatErrorReason,
-		queryClient,
 		scheduleStreamReset,
 		setChatErrorReason,
 		store,

site/src/pages/AgentsPage/AgentsPage.tsx

@@ -2,6 +2,8 @@ import { watchChats } from "api/api";
 import { getErrorMessage } from "api/errors";
 import {
 	archiveChat,
+	chatDiffContentsKey,
+	chatDiffStatusKey,
 	chatKey,
 	chatModelConfigs,
 	chatModels,
@@ -291,6 +293,21 @@ const AgentsPage: FC = () => {
 				return;
 			}
 
+			if (chatEvent.kind === "diff_status_change") {
+				void Promise.all([
+					queryClient.invalidateQueries({
+						queryKey: chatsKey,
+					}),
+					queryClient.invalidateQueries({
+						queryKey: chatDiffStatusKey(updatedChat.id),
+					}),
+					queryClient.invalidateQueries({
+						queryKey: chatDiffContentsKey(updatedChat.id),
+					}),
+				]);
+				return;
+			}
+
 			queryClient.setQueryData(
 				chatsKey,
 				(prev: TypesGen.Chat[] | undefined) => {