nisse-dev: a VM lost as a helper's work is taken back ends the run

The property walk (seed 468, a CI run's moving seed) found it: when the
build VM went out of reach while _delegate moved the helper's work back
onto the milestone's branch, git's HostUnavailable came back as a
GitError, was noted as a fact, and the driver's run went on, calling the
model with its VM gone (no_model_on_a_lost_vm). Now a GitError caused by a
lost VM raises VMUnreachable once the delegation's intents are settled, so
the run ends and the VM is blocked, as for any other lost-VM call (#137).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J5Utyfzw7gF7UhjRDEtjTk

nisse-dev/tests/test_loop_vm.py

@@ -338,3 +338,42 @@ def test_a_vm_lost_just_before_the_step_blocks_before_any_model_call(
     agent, _ = agent_for(world, project, model, clock=Clock())
     agent.intake()
     assert list(blocks(world).values()) == ["unreachable"]
+
+
+def test_a_vm_lost_as_a_helper_hands_back_ends_the_drivers_run(
+    world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    # Found by the property walk (seed 468): taking the helper's work back
+    # onto the milestone's branch failed for the lost VM, was noted as a git
+    # error, and the driver's model was called with the VM gone.
+    from test_loop_help import Spy, hand_back, helpers
+
+    faults = Faults(monkeypatch, world["host"])
+
+    class Leaving(Spy):
+        def complete(self, events, tools, model, system=None) -> list[Any]:
+            out = super().complete(events, tools, model, system)
+            if not self.turns:  # gone as its work is taken back
+                faults.drop_on = "checkout -q -f -B"
+            return out
+
+    helper = Leaving(
+        [call("write_file", path="helped.py", content="H = 1\n")], [hand_back()]
+    )
+    model = ScriptedModel(
+        ready(),
+        RESTATE,
+        [
+            call("start_milestone", name="m"),
+            call("delegate", task="write helped.py"),
+        ],
+        [call("run_command", command="echo never")],  # never asked for
+    )
+    agent, transport = agent_for(
+        world, project, model, clock=Clock(), helpers=helpers(helper)
+    )
+    agent.intake()
+    say(transport, world, "go")
+    assert list(blocks(world).values()) == ["unreachable"]
+    assert the_item(world).position == READY
+    assert len(model.turns) == 1  # the driver's model wasn't called again