hive-c0re: back agent state dirs with btrfs subvolumes

Progressive enhancement: a brand-new agent's state root under
/var/lib/hyperhive/agents is created as a btrfs subvolume when the host
filesystem is btrfs, otherwise it falls back to a plain directory. No
existing agent is auto-migrated — the new path only fires when the root
does not yet exist, so plain-dir agents are left untouched until an
explicit opt-in upgrade.

Two new privileged ops (subvolume create/delete are root-only):
EnsureAgentSubvolume statfs-gates on btrfs, creates the subvolume, and
chowns it to the hive-core user so the normal state/claude/harness
mkdirs succeed inside it; DeleteAgentSubvolume btrfs-subvolume-deletes
the root iff it is actually a subvolume. hive-c0re calls Ensure before
the per-agent dirs are created (spawn/rebuild/InitConfig) and Delete on
the purge path only — destroy keeps the subvolume for revival, matching
plain-dir semantics. btrfs-progs added to the hive-priv unit PATH.

Per-subvolume usage accounting + optional quota is a separate
follow-up.

nix/modules/hive-c0re.nix

@@ -930,6 +930,7 @@ in
         pkgs.nix # nix, nix-env, nix-instantiate — create + update
         pkgs.util-linux # umount (nsenter is hardcoded in the script)
         pkgs.e2fsprogs # chattr
+        pkgs.btrfs-progs # btrfs subvolume create/delete — Ensure/DeleteAgentSubvolume
       ];
       environment = {
         # `nixos-container update/create` runs `nix`, which writes its