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.

tests/test_extension_base.py

@@ -1,5 +1,7 @@
 """Extension SDK base class tests."""
 
+import pytest
+
 from sandboxer.extensions.base import SandboxExtension
 from sandboxer.extensions.compose_ssh import ComposeSSHExtension
 from sandboxer.extensions.vm_packer import VMPackerExtension
@@ -13,4 +15,21 @@ def test_reference_extensions_subclass_base() -> None:
 def test_new_sandbox_id_from_inputs() -> None:
     assert SandboxExtension.new_sandbox_id({"sandbox_id": "fixed123"}) == "fixed123"
     generated = SandboxExtension.new_sandbox_id({})
-    assert len(generated) == 8
+    assert len(generated) == 8
+
+
+def test_default_snapshot_not_supported() -> None:
+    class MinimalExtension(SandboxExtension):
+        def provision(self, profile, inputs, host):
+            return {}
+
+        def wait_ready(self, handle):
+            return {}
+
+        def teardown(self, handle):
+            return {}
+
+    ext = MinimalExtension()
+    assert ext.supports_snapshots() is False
+    with pytest.raises(NotImplementedError):
+        ext.snapshot({})