SendAgentSnapshotToFile priv op: btrfs send [-p <parent>] <snapshot> to a
file under MIGRATE_STAGING_ROOT. Standalone-useful as a point-in-time
snapshot export/backup today; the cross-hive ssh-piped leg (auth/trust
design posted on #1763, awaiting mara/damocles steer) is a later,
separate piece this doesn't block on.
- hive-sh4re: PrivRequest::SendAgentSnapshotToFile + MIGRATE_STAGING_ROOT
- hive-priv: validates names, refuses to overwrite an existing export,
cleans up a partial file on btrfs send failure
- hive-c0re: priv_client::send_agent_snapshot_to_file
- hivectl: `hivectl subvol snapshot send <agent> <label> [--parent <label>] --dest <file>`
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