chore: add gmail hook setup notes

src/hooks/gmail-ops.ts

@@ -114,6 +114,7 @@ export async function runGmailSetup(opts: GmailSetupOptions) {
     opts.project ??
     parsedTopic?.projectId ??
     (await resolveProjectIdFromGogCredentials());
+  // Gmail watch requires the Pub/Sub topic to live in the OAuth client project.
   if (!projectId) {
     throw new Error(
       "GCP project id required (use --project or ensure gog credentials are available)",