Step 10: GetManifest, BlobExists, ReadBlob, WriteBlob, ReplaceManifest
accessor methods on Garden. 5 tests.
Step 11: ManifestToProto/ProtoToManifest conversion functions in
server package with time.Time <-> timestamppb handling. Round-trip
test covering all 3 entry types.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
