When editing backup metadata schema in proto/brpb.proto, use:
.agents/skills/backup-meta-schema-version/SKILL.md.
related structures:
BackupMetaBackupRangeTableMetaFileMetaFilePlacementPolicyStatsFileIndexSchemaIDMapPitrTableMapPitrDBMapRawRange
Any wire-format/semantics change to the messages above requires
BackupSchemaVersion to be incremented in the same PR
(pkg/brpb/backup_schema_version.go).