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.

hive-c0re/src/lifecycle.rs

@@ -878,7 +878,7 @@ pub fn ensure_state_dir(notes_dir: &Path) -> Result<()> {
 /// filesystem supports it — BEFORE the per-agent subdirs (`state/`, `claude/`,
 /// `harness/`) are created by `ensure_state_dir` / `ensure_claude_dir`.
 ///
-/// Progressive enhancement (the #1762 model): if the root already exists
+/// Progressive enhancement: if the root already exists
 /// (any agent provisioned before this landed, plain dir or subvol) it's left
 /// exactly as-is — no auto-migration — and the priv round-trip is skipped. On
 /// a non-btrfs host the priv op no-ops and the root is later created as a