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/state.py

@@ -98,7 +98,7 @@ class VMBlock:
     project on it waits."""
 
     host: str
-    cause: str  # broken (the driver said so); unreachable (#137)
+    cause: str  # broken (the driver said so), unreachable, host key (#137)
     reason: str
     work_item: str | None  # the item that found it
     since: str
@@ -411,6 +411,15 @@ class StateStore:
         if row is not None:
             self.oplog.emit("vm.block", cause=cause, host=host, work_item=work_item)
 
+    def reblock_vm(self, host: str, *, cause: str, reason: str) -> None:
+        """A blocked VM's block, for another cause (an unreachable VM that
+        answers with a new host key)."""
+        self._conn.execute(
+            "UPDATE vm_blocks SET cause = ?, reason = ? WHERE host = ?",
+            (cause, reason, host),
+        )
+        self.oplog.emit("vm.block", cause=cause, host=host)
+
     def unblock_vm(self, host: str, *, cause: str) -> VMBlock | None:
         """Lift a build VM's block; returns it, or None if there was none."""
         row = self._conn.execute(