Implements the api-layer half of #45. UpdateNote supersedes a note in
place (whole-note body replace, frontmatter re-stamp: updated_at,
supersedes=prior content hash, supersede_reason), preserving created_at,
wing, hall, and any custom fields. Never creates — a missing target is
an error so callers fall back to brain_write. ReadNote is the read-after-
write primitive (frontmatter + body + content_hash). ContentHash is the
sha256 handle that round-trips write/update → get.
Frontmatter is edited via a line-preserving ordered editor rather than a
yaml.v3 round-trip, which would reorder keys and strip comments — the
brain writes flat key:value frontmatter by hand.
Embeddings are not refreshed here: the rewritten file's mtime advances,
which the out-of-band vectorstore.Sync ticker uses to re-embed it — the
same mechanism brain_write relies on.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
