btrfs subvols: scrub tracker tags from comments; harden subvol chown

- Replace the #-number tracker references in code comments with prose
  (tracker-tag lint; hive convention is prose in source).
- ensure_agent_subvolume now treats a chown failure on the freshly
  created subvolume as fatal: it rolls the subvolume back (deletes it)
  and returns an error, instead of warning and leaving a root-owned
  subvol that hive-c0re can't write into (which would also make the
  c0re-side exists-check skip the retry, wedging the agent).

hive-c0re/src/actions.rs

@@ -660,7 +660,7 @@ pub async fn destroy(coord: &Arc<Coordinator>, name: &str, purge: bool) -> Resul
         let _ = std::fs::remove_dir_all(&runtime);
     }
     if purge {
-        // The state root may be a btrfs subvolume (#1762): a subvolume root
+        // The state root may be a btrfs subvolume: a subvolume root
         // can't be removed with rmdir/`remove_dir_all`, so delete it via
         // hive-priv (root) first. No-op for plain-dir agents — the loop below
         // then handles the plain-dir state root plus the applied dir.