ci: keep pinned-config swap invisible to git in merge-queue job

The cp on its own left .mdsmith.yml dirty in the runner's
checkout; merge-queue-action performs local checkout/merge
operations after the merge driver is installed, and git would
either refuse those with "local changes would be overwritten"
or silently restore the new-shape config mid-merge — at which
point the pinned binary stops being able to parse it.

`git update-index --skip-worktree .mdsmith.yml` after the cp
tells git to treat the file as unchanged. `git status` reports
the worktree clean, branch switches and patch applies leave
the swapped content alone, and the merge driver continues to
see the pre-156 shape for the whole job lifetime.

https://claude.ai/code/session_012GGH62fZUzLuzP8T4ocGkJ

Author: claude

.github/workflows/merge-queue.yml

@@ -42,11 +42,18 @@ jobs:
       # merge) does the same. v0.15.0 cannot parse plan 156's
       # inline schema vocabulary, so the workflow swaps in
       # .mdsmith.pinned.yml — a config restated in the pre-156
-      # shape — before the install / merge steps run. Remove this
-      # swap once setup-mdsmith-pinned-version advances past
-      # v0.15.0 (and delete .mdsmith.pinned.yml).
+      # shape — for the entire job lifetime. `update-index
+      # --skip-worktree` keeps the swap invisible to git so the
+      # later checkout/merge operations performed by
+      # merge-queue-action don't trip "local changes would be
+      # overwritten" or try to restore the new-shape config
+      # mid-merge. Remove this block once
+      # setup-mdsmith-pinned-version advances past v0.15.0 (and
+      # delete .mdsmith.pinned.yml).
       - name: Use pinned-binary config for merge driver
-        run: cp .mdsmith.pinned.yml .mdsmith.yml
+        run: |
+          cp .mdsmith.pinned.yml .mdsmith.yml
+          git update-index --skip-worktree .mdsmith.yml
       - name: Install mdsmith merge driver and pre-merge-commit hook
         run: |
           mdsmith merge-driver install