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.

hive-c0re/src/priv_client.rs

@@ -305,6 +305,27 @@ pub async fn control_infra_container(container: &str, action: InfraAction) -> Re
     .await?)
 }
 
+/// Ensure the agent's persistent state root is a btrfs subvolume when the
+/// host FS supports it (via hive-priv, which runs as root). Idempotent and
+/// progressive: a no-op when the root already exists or the FS isn't btrfs.
+/// Safe to call on every provision.
+pub async fn ensure_agent_subvolume(agent_name: &str) -> Result<()> {
+    ok(call(&PrivRequest::EnsureAgentSubvolume {
+        agent_name: agent_name.to_owned(),
+    })
+    .await?)
+}
+
+/// Delete the agent's state root iff it is a btrfs subvolume (purge path
+/// only). No-op for plain dirs / missing paths — hive-c0re's own
+/// `remove_dir_all` handles those.
+pub async fn delete_agent_subvolume(agent_name: &str) -> Result<()> {
+    ok(call(&PrivRequest::DeleteAgentSubvolume {
+        agent_name: agent_name.to_owned(),
+    })
+    .await?)
+}
+
 fn check(resp: PrivResponse) -> Result<(String, String)> {
     if resp.ok {
         Ok((resp.stdout, resp.stderr))