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_live_fixes.py

@@ -156,6 +156,34 @@ def test_a_review_comment_can_be_answered_without_a_fix(
     assert "It's a constant on purpose" in forge.posted[-1][1]
 
 
+def test_a_remark_waiting_to_be_answered_doesnt_outlive_the_merge(
+    world: dict[str, Any],
+) -> None:
+    """#137's walk: a review comment readies the item to answer it and go back
+    to waiting for the merge; if the merge comes before that run (a blocked
+    VM, a queue), there is no merge left to go back to."""
+    forge: Forge = world["forge"]
+    model = ScriptedModel(
+        RESTATE,
+        [
+            call("start_milestone", name="words"),
+            call("write_file", path="w.py", content="W = 1\n"),
+            call("finish_milestone", summary="words", final="yes"),
+        ],
+    )
+    agent, transport = loop_agent(world, model)
+    agent.intake()
+    say(transport, world, "go")
+    [pr] = forge.prs
+    forge.comments[pr] = [Comment(7, "don", "why a constant", "t")]
+    agent.watch()  # not run yet
+    assert loop_of(the_item(world)).back_to == MERGE
+    forge.merge(pr)
+    agent.watch()
+    loop = loop_of(the_item(world))
+    assert (loop.pr, loop.back_to, loop.back_note) == (0, "", "")
+
+
 # --- the session's later findings ----------------------------------------------