feat: snapshot/restore checkpoints (SAND-WP-0007)

Add workspace checkpoint API with SnapshotStore, extension hooks on
compose-ssh and saas-stub, manager orchestration, CLI/HTTP surface,
profile.compose-checkpoint, and docs/tests.

src/sandboxer/snapshots/__init__.py

@@ -0,0 +1,5 @@
+"""Snapshot checkpoint persistence."""
+
+from sandboxer.snapshots.store import SnapshotStore
+
+__all__ = ["SnapshotStore"]