A design milestone holds documents only, enforced

Found by the first local e2e run: qwen3.8-27b put a new project's code in
the same PR as its ARCHITECTURE.md, with no build plan. D1 says a `design`
item agrees its design before any code, and that rested on wording only.

- Until a PR of a `design` item has merged, finish_milestone is sent back
  if the milestone changes anything but ARCHITECTURE.md and docs/, naming
  the paths to take out. The merge is recorded (design_merged) and code
  may follow.
- The label's instruction and the new project's brief say so plainly:
  documents first, in a PR of their own; code after it has merged.

Tests: a design milestone with code is sent back and only the documents
are published, then code after the merge; the walk's model follows the
rule.

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

nisse-dev/tests/test_loop_properties.py

@@ -63,6 +63,11 @@ class PolicyModel:
         self.n += 1
         offered = {t["function"]["name"] for t in tools}
         facts = [getattr(e.event, "text", "") for e in events]
+        # A `design` item writes documents until its design has merged.
+        design = "labelled design" in (system or "") and not any(
+            "the design has merged" in t for t in facts
+        )
+        path = f"docs/d{self.n}.md" if design else f"f{self.n}.py"
         roll = self.rng.random()
         if roll < 0.08:
             return [call("ask_human", question=f"question {self.n}?")]
@@ -98,7 +103,7 @@ class PolicyModel:
             # Half done, then a question: work a switch must park.
             return [
                 call("start_milestone", name=f"m{self.n}"),
-                call("write_file", path=f"f{self.n}.py", content=f"x = {self.n}\n"),
+                call("write_file", path=path, content=f"x = {self.n}\n"),
                 call("ask_human", question=f"halfway {self.n}?"),
             ]
         merged = any("merged" in t for t in facts[-6:])
@@ -106,7 +111,7 @@ class PolicyModel:
             return [call("close_item", summary=f"done {self.n}")]
         return [
             call("start_milestone", name=f"m{self.n}"),
-            call("write_file", path=f"f{self.n}.py", content=f"x = {self.n}\n"),
+            call("write_file", path=path, content=f"x = {self.n}\n"),
             call(
                 "finish_milestone",
                 summary=f"work {self.n}",