git notes attaches metadata to commits via refs/notes/<name>. Some tools (CI, code review tooling) rely on notes; we should be able to read and write them.

Scope

• Notes.read(repo, notes_ref \\ \"refs/notes/commits\", commit_sha) → binary or :none.
• Notes.write(repo, notes_ref, commit_sha, content, committer_opts) → new notes-tree sha.
• Handle notes-tree fan-out layout (git uses 2+2 fanout directories).

Acceptance

• Round-trip: git notes add -m \"hello\" <sha>, read via our API, get \"hello\".
• Write our own note, verify via git notes show.