New agents get a btrfs subvolume state root automatically when the host
FS is btrfs, but agents that predate that migration are left on plain
dirs and miss the subvolume feature set (snapshots, per-subvol
usage/quota, send/receive migration). Add an opt-in operator verb to
convert an existing plain-dir agent in place.
btrfs cannot promote a directory to a subvolume in place, so the new
privileged op stages a sibling subvolume mirroring the dir (create +
`cp -a --reflink=auto` preserving ownership/permissions/xattrs + match
the root's owner and mode), then atomically renames the original aside
and the subvolume into place, then removes the original. Any failure
before the swap leaves the original untouched; idempotent (no-op if
already a subvolume) and btrfs-gated.
The `hivectl subvol upgrade <agent> --yes` verb composes it client-side
like `restart`: stop the agent so its state bind-mount is released, run
the migration via hive-priv, then restart it — the restart is attempted
regardless of the migration outcome so a failed migration never leaves
the agent down.
- hive-sh4re: UpgradeAgentSubvolume priv request variant.
- hive-priv: the migration handler plus stage/cleanup helpers.
- hive-c0re: priv_client wrapper and the hivectl verb; regen CLI docs.
6b1dbebe5a