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/src/nisse_dev/loop/agent.py

@@ -150,9 +150,11 @@ LABELS = {
     "discuss": "This issue is labelled discuss: talk it through with the "
     "operator (ask_human) before restating, and restate only once you agree "
     "on what is wanted.",
-    "design": "This issue is labelled design: agree the design with the "
-    "operator before any code. Your restatement puts the design to them; "
-    "code waits for their go-ahead on it.",
+    "design": "This issue is labelled design: the design comes before any "
+    "code. Your first milestone is documents only: ARCHITECTURE.md and a "
+    "build plan under docs/plans/, in a PR of their own (if you restate, put "
+    "the design to the operator). Code starts in a later milestone, after that "
+    "PR has merged; the harness refuses a design milestone with anything else.",
 }
 
 
@@ -519,6 +521,9 @@ class LoopAgent:
             if pr.merged:
                 self._fact(item, f"PR #{loop.pr} merged")
                 loop, position = replace(loop, pr=0, pr_url="", branch=""), READY
+                if "design" in loop.labels and not loop.design_merged:
+                    self._fact(item, "the design has merged: code may follow")
+                    loop = replace(loop, design_merged=True)
             elif pr.state != "open":
                 self._fact(item, f"PR #{loop.pr} was closed without merging")
                 loop, position = replace(loop, pr=0, pr_url="", branch=""), READY
@@ -1047,7 +1052,8 @@ class LoopAgent:
                 project.repo,
                 title,
                 f"{brief}\n\nFrom a conversation with the operator, agreed there "
-                f"as rev-{loop.rev}. First: ARCHITECTURE.md and the build plan."
+                f"as rev-{loop.rev}. First: ARCHITECTURE.md and the build plan, "
+                "in a PR of documents only; code after it has merged."
                 f"\n\n<!-- nisse-item: {new_id} -->",
             )
             self._gitea.assign_issue(project.repo, number, self._account)
@@ -1211,6 +1217,32 @@ class LoopAgent:
                 )
         self._last_project = here
 
+    def _design_problem(self, item_id: str) -> str | None:
+        """D1, D2: a `design` item agrees its design before any code. Until
+        a PR of it has merged, a milestone holds documents only."""
+        item = self._store.work_item(item_id)
+        if item is None:
+            return None
+        loop = loop_of(item)
+        milestones = self._at(item).milestones
+        if "design" not in loop.labels or loop.design_merged or milestones is None:
+            return None
+        try:
+            changed = milestones.changed()
+        except GitError:
+            return None  # publishing reports what's wrong with the workspace
+        code = [
+            p for p in changed if p != "ARCHITECTURE.md" and not p.startswith("docs/")
+        ]
+        if not code:
+            return None
+        return (
+            "not published: this milestone is the design, so it holds documents "
+            "only (ARCHITECTURE.md, docs/); code starts in a later milestone, "
+            "after this one's PR has merged. Take these out of it (they can "
+            "wait in the plan): " + ", ".join(code)
+        )
+
     def _converse_run(self, item: WorkItem) -> None:
         """A conversation's turn (L1, L5): it talks, and may propose work on
         a project. No workspace, nothing to run."""
@@ -1329,7 +1361,10 @@ class LoopAgent:
                 tools=tools,
                 workspace=workspace,
                 handlers={"start_milestone": lambda args: self._start(item.id, args)},
-                checks={"restate": _restatement_problem},
+                checks={
+                    "restate": _restatement_problem,
+                    "finish_milestone": lambda args: self._design_problem(item.id),
+                },
             )
         finally:
             self._running = False