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/milestones.py

@@ -161,6 +161,10 @@ class Milestones:
         if git.remote_sha(self._remote, parked, credential=self._credential):
             git.delete_remote_branch(self._remote, parked, credential=self._credential)
 
+    def changed(self) -> list[str]:
+        """What the current milestone changes, from the default branch."""
+        return self._git.changed_paths(f"origin/{self._base}")
+
     # -- a new project's checks (D1, D4) -----------------------------------------
 
     def verify_access(self, account: str) -> list[str]: