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/src/nisse_dev/loop/agent.py

@@ -2111,6 +2111,7 @@ class LoopAgent:
         result = delegation.run(
             self._store.transcript(item.id, "helper"), brief=brief, branch=branch
         )
+        lost: GitError | None = None
         try:
             # The milestone takes the helper's work (or, on failure, only
             # its own back) as uncommitted changes: harness branches never
@@ -2122,6 +2123,8 @@ class LoopAgent:
                 git._git("branch", "-q", "-D", branch)
         except GitError as exc:
             self._fact(item, f"after helper {number}: {exc}")
+            if isinstance(exc.__cause__, HostUnavailable):
+                lost = exc  # the VM went: the driver's run ends too (#137)
         for intent in self._store.pending_intents():
             # Cut off rather than finished: discarded all the same, never
             # resumed blindly (L8, L9).
@@ -2130,6 +2133,8 @@ class LoopAgent:
                 and intent.payload.get("branch") == branch
             ):
                 self._store.abort_intent(intent.id, cause=f"delegation {result.status}")
+        if lost is not None:
+            raise VMUnreachable(str(lost)) from lost
         if result.usable and result.handback is not None:
             self._fact(item, f"helper {number} finished: {result.handback.render()}")
             return f"the helper finished:\n{result.handback.render()}", True