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/config.py

@@ -137,6 +137,15 @@ class LoopTuning:
     general_idle_s: float = 86400.0
 
 
+@dataclass(frozen=True)
+class BuildVMTuning:
+    # An unreachable build VM is tried again this often (#137).
+    retry_s: float = 300.0
+    # Once it answers, it must still answer this much later before work
+    # goes on, so it has finished booting.
+    settle_s: float = 180.0
+
+
 @dataclass(frozen=True)
 class Tuning:
     driver: DriverTuning = field(default_factory=DriverTuning)
@@ -146,6 +155,7 @@ class Tuning:
     tools: ToolsTuning = field(default_factory=ToolsTuning)
     context: ContextTuning = field(default_factory=ContextTuning)
     loop: LoopTuning = field(default_factory=LoopTuning)
+    build_vm: BuildVMTuning = field(default_factory=BuildVMTuning)
 
 
 _TUNING_SECTIONS: dict[str, type] = {