M4.1: toolchain preflight

A model works out and sets up what a project needs on its build VM;
code makes sure that happens, bounds it and checks the result (D3).

- Before each driver run on a project, one round trip checks the build
  VM: the harness's own baseline (missing: the item waits, a template
  problem), the workspace clone (missing: cloned again and said; the
  item resumes from what it pushed) and a per-project marker.
- No marker (a new project, a new or wiped VM): the item's driver model
  runs a bounded preflight first, with its own prompt, the execution
  tools (screened as ever) and toolchain_ready. The harness runs each
  reported version command itself (version-shaped commands only, even
  from the marker, which the model's commands could edit) and writes
  the marker. Gaps and an unfinished preflight are facts and work goes
  on; the thread hears only of installs and gaps. A rebuilt VM starts
  from the last toolchain.
- Later runs check the versions again; a change is a fact and a thread
  line. The PR body says what the work was tested with.
- A changed host key is its own failure (HostKeyChanged): the item
  waits for the operator's `trust new host key`, which drops the pin;
  then the VM is set up again.
- A prompt line: install what the work needs on the build VM, say why,
  and put lasting needs in the project's own pins.
- tuning.preflight: max_turns, command_timeout_s.
- The property walk gains a VM wipe and ready_before_work (checked
  against a mutant that skips preflight); parked_work_safe accepts that
  a wipe loses what wasn't pushed (D3).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

nisse-dev/src/nisse_dev/loop/items.py

@@ -77,6 +77,11 @@ class Loop:
     # The trust dial (M3.3): why an approved PR was last left unmerged, so
     # 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.
+    toolchain: tuple[tuple[str, str, str], ...] = ()
+    host_key_changed: bool = False
 
     def to_data(self) -> dict[str, Any]:
         return asdict(self)
@@ -88,6 +93,7 @@ def loop_of(item: WorkItem) -> Loop:
     data["blocked_by"] = tuple(data.get("blocked_by", ()))
     data["labels"] = tuple(data.get("labels", ()))
     data["became"] = tuple(data.get("became", ()))
+    data["toolchain"] = tuple(tuple(t) for t in data.get("toolchain", ()))
     return Loop(**data)