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/items.py

@@ -55,6 +55,7 @@ class Loop:
     proposal_title: str = ""
     proposal_kind: str = ""  # "work" on a project, or a new "project"
     new_project: str = ""  # a new project being provisioned (D1)
+    design_merged: bool = False  # a `design` item's design PR has merged (D1, D2)
     # Where the operator last spoke (D5: answer where asked): "" for the
     # Matrix thread, or the PR it came from, with the inline location and a
     # little of what they said, for quoting.