Follow-up to the btrfs-subvolume migration. Operator-opt-in disk
accounting + quotas on agent state subvolumes via btrfs qgroups:
- three privileged ops (qgroup ops need root): EnsureBtrfsQuota
(btrfs quota enable on the agent-state filesystem — statfs-gated,
idempotent, no-op off btrfs), ReadSubvolumeUsage (btrfs qgroup show
-f --raw for one agent), SetSubvolumeQuota (btrfs qgroup limit, or
clear). Reuses the is_on_btrfs helper from the subvolume work.
- priv_client wrappers, incl parse_qgroup_usage -> (referenced,
exclusive) bytes.
- hivectl 'quota' subcommand: enable / show [agent] / limit <agent>
<size|none>, with a K/M/G/T size parser + human-readable output.
Quota is deliberately NOT auto-enabled: btrfs quota enable triggers a
full rescan that is I/O-heavy on a large filesystem, and the operator
should choose when to pay that. 'quota show' on a plain-dir agent (no
subvolume) reports no qgroup data rather than erroring.
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