docs(snapshot-store): cross-link the state subvolume to persistence.md

docs/snapshot-store.md

@@ -183,7 +183,10 @@ the access-review list.
 ### What a snapshot contains
 
 The snapshot covers an agent's **state subvolume**, which is the parent
-of `state/`, `claude/` and `harness/`. Consequences:
+of `state/`, `claude/` and `harness/` (see
+[`docs/persistence.md`'s btrfs subvolume
+section](persistence.md#btrfs-subvolumes-for-varlibhyperhiveagentsname)
+for how and when that subvolume is created). Consequences:
 
 - The Claude session (`claude/`) travels, so a restored agent keeps its
   live `--continue` session rather than needing to log in again.