Download a chat's whole workspace as a zip

Streams every file in the running chat agent’s /workspace as a zip archive using safe relative POSIX entry names. Production follows response backpressure and aborts in-flight sandbox downloads if the consumer disconnects, so the whole archive is never buffered. Export is rejected before streaming when the workspace listing exceeds 2 MiB or 10,000 entries, any file is over 50 MiB, or advertised content exceeds 512 MiB total. Each file response is bounded before JSON parsing and base64 decoding, and the stream fails instead of returning a partial archive if a file grows beyond 50 MiB after listing, actual total content exceeds 512 MiB, a file cannot be read, or the 30-second archive deadline expires.