M4.4: an unreachable build VM blocks its projects until it's back (#137)

M4.3's block, with new triggers and a way out. A VM out of reach at the
check before a run, in a run (which ends there: the driver, a toolchain
step and a requested step), or while preparing the workspace blocks every
project on it; the item stays ready, waiting on no one. On the intake round
it is tried again every tuning.build_vm.retry_s (300), and once it answers
it must still answer settle_s (180) later; then the work a lost run left
there is parked (pushed), the VM unblocked, and that said once.

A changed host key, at the check or on a retry, blocks the VM too, until
the operator's `vm ready`, which trusts the new key: `trust new host key`
is retired, and the message says it's a key problem that may be an
impersonation. Questions are answered while a VM is blocked, without it.
Starting the agent with a VM down no longer fails.

The walk breaks the VM's SSH, now or partway through a run, and checks that
no model works on a VM that is down; loop.qnt models the VM going and
coming back (unblocked_only_when_up). Together they found: a lost run's
work set aside by the next item there; a question left behind a block; a
lost VM swallowed while the toolchain step checked or recorded it; and a
remark waiting to be answered that outlived its PR's merge. All fixed.

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.py

@@ -24,6 +24,7 @@ from nisse_dev.loop.items import CLOSED, GO_AHEAD, READY, RUNNING, WAITING, loop
 from nisse_dev.loop.reader import command_form
 from nisse_dev.oplog import OpLog
 from nisse_dev.state import StateStore
+from nisse_dev.tools import ToolOutcome
 from nisse_dev.transport import InMemoryTransport
 from nisse_dev.worker import InlineWorker
 
@@ -430,7 +431,7 @@ def test_a_command_that_works_resets_the_failure_count(tmp_path: Path) -> None:
     class Commands:
         def dispatch(self, tool: str, arguments: dict[str, Any]) -> Any:
             ok = arguments["command"] == "good"
-            return type("Outcome", (), {"result": "out", "success": ok})()
+            return ToolOutcome("out", ok)
 
     def run(command: str) -> list[Any]:
         return [call("run_command", command=command)]