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_config.py

@@ -167,6 +167,7 @@ def test_tuning_is_read() -> None:
             "delegation": {"max_turns": 20, "max_tokens": 100000, "wall_clock_s": 600},
             "tools": {"command_timeout_s": 300, "max_output_chars": 2000},
             "loop": {"bug_grace_s": 120},
+            "build_vm": {"retry_s": 60},
         }
     )
     tuning = config.tuning
@@ -176,6 +177,7 @@ def test_tuning_is_read() -> None:
     assert tuning.delegation.wall_clock_s == 600.0
     assert tuning.tools.max_output_chars == 2000
     assert tuning.loop.bug_grace_s == 120.0
+    assert (tuning.build_vm.retry_s, tuning.build_vm.settle_s) == (60.0, 180.0)
 
 
 def test_a_partly_given_tuning_section_keeps_the_other_defaults() -> None: