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/src/nisse_dev/loop/items.py

@@ -78,10 +78,8 @@ class Loop:
     # it's said once per commit and reason.
     merge_held: str = ""
     # The toolchain as the harness last checked it (M4.1): name, version
-    # command, version; and a build VM whose host key changed, waiting on
-    # the operator's word.
+    # command, version.
     toolchain: tuple[tuple[str, str, str], ...] = ()
-    host_key_changed: bool = False
 
     def to_data(self) -> dict[str, Any]:
         return asdict(self)
@@ -89,6 +87,8 @@ class Loop:
 
 def loop_of(item: WorkItem) -> Loop:
     data = dict(item.data.get("loop", {}))
+    # Retired: a changed host key now blocks the build VM (#137).
+    data.pop("host_key_changed", None)
     data["notes"] = tuple(data.get("notes", ()))
     data["blocked_by"] = tuple(data.get("blocked_by", ()))
     data["labels"] = tuple(data.get("labels", ()))