feat(#2391): btrfs SnapshotAgentSubvolume/DeleteAgentSnapshot priv ops

Adds the first missing piece from #2391's migration-gaps list: a
read-only btrfs snapshot priv op so hivectl migrate can freeze a
consistent point-in-time copy of an agent's state subvolume for
btrfs send, without stopping the live agent.

- PrivRequest::SnapshotAgentSubvolume / DeleteAgentSnapshot (hive-sh4re)
- hive-priv handlers: btrfs subvolume snapshot -r / delete, sibling
  dot-prefixed path (<AGENT_STATE_ROOT>/.<agent>.snapshot.<label>)
- hive-c0re::priv_client wrappers
- hivectl subvol snapshot / delete-snapshot verbs (no agent stop needed
  — btrfs snapshots are atomic against a live subvolume)

Does not yet wire actual btrfs send/receive or the hivectl migrate
verb — those stay tracked on #2391 as separate follow-up pieces.

hive-priv/src/main.rs

@@ -365,6 +365,24 @@ async fn exec(req: PrivRequest, writer: &mut OwnedWriteHalf) -> Result<(String,
             upgrade_agent_subvolume(agent_name).await
         }
 
+        PrivRequest::SnapshotAgentSubvolume {
+            ref agent_name,
+            ref snapshot_name,
+        } => {
+            validate_agent_name(agent_name)?;
+            validate_credential_name(snapshot_name)?;
+            snapshot_agent_subvolume(agent_name, snapshot_name).await
+        }
+
+        PrivRequest::DeleteAgentSnapshot {
+            ref agent_name,
+            ref snapshot_name,
+        } => {
+            validate_agent_name(agent_name)?;
+            validate_credential_name(snapshot_name)?;
+            delete_agent_snapshot(agent_name, snapshot_name).await
+        }
+
         PrivRequest::SyncAgentTmpfiles { ref agents } => sync_agent_tmpfiles(agents).await,
     }
 }
@@ -1059,6 +1077,86 @@ async fn upgrade_agent_subvolume(agent_name: &str) -> Result<(String, String)> {
     ))
 }
 
+/// Derive a snapshot's path from the agent name + label: a dot-prefixed
+/// sibling of the agent's state root so it can never collide with a real
+/// agent directory (`validate_agent_name` rejects dot-prefixed names).
+fn snapshot_path(agent_name: &str, snapshot_name: &str) -> PathBuf {
+    PathBuf::from(AGENT_STATE_ROOT).join(format!(".{agent_name}.snapshot.{snapshot_name}"))
+}
+
+/// `SnapshotAgentSubvolume` — create a read-only btrfs snapshot of an
+/// agent's state subvolume, for `btrfs send` to stream from during
+/// inter-hive migration. See the wire doc.
+async fn snapshot_agent_subvolume(
+    agent_name: &str,
+    snapshot_name: &str,
+) -> Result<(String, String)> {
+    let agent_root = PathBuf::from(AGENT_STATE_ROOT).join(agent_name);
+    if !is_btrfs_subvolume(&agent_root) {
+        bail!(
+            "{} is not a btrfs subvolume — nothing to snapshot (run `hivectl subvol upgrade` first)",
+            agent_root.display()
+        );
+    }
+    let snap = snapshot_path(agent_name, snapshot_name);
+    if snap.exists() {
+        bail!(
+            "snapshot {} already exists — delete it first or pick a different name",
+            snap.display()
+        );
+    }
+    let out = Command::new("btrfs")
+        .args(["subvolume", "snapshot", "-r"])
+        .arg(&agent_root)
+        .arg(&snap)
+        .output()
+        .await
+        .with_context(|| {
+            format!(
+                "spawn btrfs subvolume snapshot -r {} {}",
+                agent_root.display(),
+                snap.display()
+            )
+        })?;
+    if !out.status.success() {
+        bail!(
+            "btrfs subvolume snapshot -r {} {} failed: {}",
+            agent_root.display(),
+            snap.display(),
+            String::from_utf8_lossy(&out.stderr).trim()
+        );
+    }
+    tracing::info!(
+        agent = %agent_name, snapshot = %snap.display(),
+        "created read-only agent state snapshot"
+    );
+    Ok((snap.display().to_string(), String::new()))
+}
+
+/// `DeleteAgentSnapshot` — delete a previously-created read-only snapshot.
+/// No-op if the path doesn't exist. See the wire doc.
+async fn delete_agent_snapshot(agent_name: &str, snapshot_name: &str) -> Result<(String, String)> {
+    let snap = snapshot_path(agent_name, snapshot_name);
+    if !snap.exists() {
+        return Ok((String::new(), String::new()));
+    }
+    let out = Command::new("btrfs")
+        .args(["subvolume", "delete"])
+        .arg(&snap)
+        .output()
+        .await
+        .with_context(|| format!("spawn btrfs subvolume delete {}", snap.display()))?;
+    if !out.status.success() {
+        bail!(
+            "btrfs subvolume delete {} failed: {}",
+            snap.display(),
+            String::from_utf8_lossy(&out.stderr).trim()
+        );
+    }
+    tracing::info!(agent = %agent_name, snapshot = %snap.display(), "deleted agent state snapshot");
+    Ok((String::new(), String::new()))
+}
+
 /// `SetSubvolumeQuota` — set or clear a qgroup size limit on an agent
 /// subvolume (`btrfs qgroup limit <bytes|none> <…/agent_name>`). See the
 /// wire doc.