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-c0re/src/bin/hivectl.rs

@@ -620,6 +620,24 @@ enum SubvolCmd {
         #[arg(long)]
         yes: bool,
     },
+    /// Create a read-only snapshot of an agent's state subvolume — the
+    /// first step of the (in-progress) inter-hive migration path, or a
+    /// manual point-in-time backup. Agent must already be a subvolume
+    /// (`subvol upgrade` first). Prints the snapshot's host path.
+    Snapshot {
+        /// Agent name (e.g. `damocles`, `iris`).
+        name: String,
+        /// Snapshot label (`[A-Za-z0-9_.-]`); defaults to a UTC timestamp.
+        #[arg(long)]
+        label: Option<String>,
+    },
+    /// Delete a snapshot created by `subvol snapshot`.
+    DeleteSnapshot {
+        /// Agent name the snapshot belongs to.
+        name: String,
+        /// Snapshot label passed to `subvol snapshot --label`.
+        label: String,
+    },
 }
 
 #[tokio::main]
@@ -693,6 +711,10 @@ async fn main() -> Result<()> {
         Cmd::Restart { scope, graceful } => restart(&socket, scope.to_scope(), graceful).await,
         Cmd::Subvol { cmd } => match cmd {
             SubvolCmd::Upgrade { name, yes } => subvol_upgrade(&socket, &name, yes).await,
+            SubvolCmd::Snapshot { name, label } => subvol_snapshot(&name, label).await,
+            SubvolCmd::DeleteSnapshot { name, label } => {
+                subvol_delete_snapshot(&name, &label).await
+            }
         },
         Cmd::Choom {
             name,
@@ -1703,6 +1725,38 @@ async fn subvol_upgrade(socket: &Path, name: &str, yes: bool) -> Result<()> {
     Ok(())
 }
 
+/// `subvol snapshot <agent> [--label <label>]` — create a read-only btrfs
+/// snapshot of an agent's state subvolume. Unlike `upgrade`, this does NOT
+/// stop the agent: btrfs snapshots are atomic + consistent to take against a
+/// live subvolume. Default label is a unix-timestamp so repeated calls don't
+/// collide without the caller having to think of a name.
+async fn subvol_snapshot(name: &str, label: Option<String>) -> Result<()> {
+    if !agent_exists(name)? {
+        bail!("no agent named {name:?} (no state dir under the agents root)");
+    }
+    let label = label.unwrap_or_else(|| {
+        let secs = std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .map_or(0, |d| d.as_secs());
+        format!("snap-{secs}")
+    });
+    let path = hive_c0re::priv_client::snapshot_agent_subvolume(name, &label)
+        .await
+        .with_context(|| format!("snapshot {name} state subvolume (label {label:?})"))?;
+    println!("{path}");
+    Ok(())
+}
+
+/// `subvol delete-snapshot <agent> <label>` — remove a snapshot created by
+/// `subvol snapshot`.
+async fn subvol_delete_snapshot(name: &str, label: &str) -> Result<()> {
+    hive_c0re::priv_client::delete_agent_snapshot(name, label)
+        .await
+        .with_context(|| format!("delete {name} snapshot (label {label:?})"))?;
+    println!("deleted snapshot {label:?} for {name}");
+    Ok(())
+}
+
 /// Render a hive-wide stop/start response: one `<verb>: <name>` line per
 /// touched container, then surface any aggregated per-target failure as a
 /// non-zero exit. `verb` is the past-tense word printed per item