perf(memfs): delta upload — only push new/modified git objects after commit
Instead of re-uploading the entire .git/ directory after every commit,
snapshot file mtimes before the commit and only upload files that are
new or changed. A typical single-block update creates ~5 new objects
(blob, trees, commit, ref) vs re-uploading all ~30.
Full _upload_repo retained for create_repo and other paths that need it.
🤖 Generated with [Letta Code](https://letta.com)
Co-authored-by: Letta <noreply@letta.com>
8f56527958