fix(record): recover failed Trace exports and hint --url when the default start page fails

Reject legacy JSON --output before recording starts, persist the completed Trace before writing the bundle so `bsk record stop --output <dir>` can retry after an export failure, and tell users to pass --url when https://example.com/ does not load.

apps/extension/src/tools/record.ts

@@ -894,7 +894,7 @@ export async function handleRecordStart(
       code: "invalid_params",
       message: params.url
         ? `cannot record on restricted URL (${startUrl}); use an http(s) page`
-        : `cannot record on restricted URL (${startUrl}); default start page must be injectable http(s)`,
+        : `cannot record on restricted URL (${startUrl}); default start page https://example.com/ did not load — pass --url with a page you can open`,
     };
   }