fix(node): pushes to mirrors of sha256-format origins parse zero ref updates (64-hex oids rejected) #511

Recorded from review of #384 (round 5), explicitly kept out of that PR as a pre-existing concern worth tracking on its own.

parse_ref_updates (crates/gitlawb-node/src/api/repos.rs) accepts only 40-hex command SHAs (parts[0].len() == 40 && parts[1].len() == 40). Repos mirrored from sha256-format origins carry 64-hex object ids, so every push to such a repo parses to zero updates:

• no receive_pack_requests / pending_ref_transitions intent rows are written,
• the refs/gitlawb/* internal-namespace gate never sees the ref names,
• the request proof is acked as vacuous (empty command stream),
• while git itself still lands the refs on disk.

This predates #384 (the parser had the same width check on main), but it means the durable post-receive tracking premise that PR establishes does not apply to that repo class — reconcile, quarantine scans, and proof-gated retention simply never see those pushes.

Suggested shape (not implemented): accept the oid width git advertises for the repo's object format (40-hex for sha1, 64-hex for sha256), or key the width off the repo's advertised object-format, plus tests for a64-hex command line both parsed and refused on a sha1 repo.