v0.18.0 — files go straight to the machine

Person uploads land on the machine. A file picked in a chat streams from the browser through the server into <working folder>/file-uploads/, verified by size and sha256, never overwriting, with a .gitignore so nothing is committed by accident. Advertised as file_uploads in hello.

Files are handed back by id, never by path. The bridge serves only files it recorded itself (received uploads, and files the assistant handed back with attach_file), re-checked at serve time, with a single Range passed through for seeking and resuming. The older path-based attachment_read is refused.

See PROTOCOL.md, "Person uploads" and "Handing files back". Servers that do not use these frames are unaffected.