feat: merge-train/barretenberg (#15336)

Co-authored-by: Jonathan Hao <jonathan@aztec-labs.com>
Co-authored-by: AztecBot <tech@aztecprotocol.com>
Co-authored-by: ledwards2225 <98505400+ledwards2225@users.noreply.github.com>

Author: 4 people

.github/workflows/merge-train-recreate.yml

@@ -16,35 +16,32 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
         with:
-          fetch-depth: 0
           token: ${{ secrets.AZTEC_BOT_GITHUB_TOKEN }}
 
       - name: Configure Git
         run: |
           git config --global user.name AztecBot
           git config --global user.email tech@aztecprotocol.com
 
-      - name: Create new merge-queue PR
+      - name: Create new merge-train PR
         env:
           GH_TOKEN: ${{ secrets.AZTEC_BOT_GITHUB_TOKEN }}
         run: |
           # The merge-train branch is the PR’s *head* branch (e.g. merge-train/main)
           MERGE_TRAIN_BRANCH="${{ github.event.pull_request.head.ref }}"
+	  BASE_BRANCH="${{ github.event.pull_request.base.ref }}"
 
-          # Make sure we have it locally
-          git fetch origin "$MERGE_TRAIN_BRANCH":"$MERGE_TRAIN_BRANCH"
-          git checkout "$MERGE_TRAIN_BRANCH"
+          # Make sure a merge commit gets into master.
+          git fetch origin "$BASE_BRANCH"
+	  git checkout "$BASE_BRANCH"
+          git merge -X ours "$MERGE_TRAIN_BRANCH" -m "Merge $MERGE_TRAIN_BRANCH history."
+	  git push origin "$BASE_BRANCH"
 
-          # Spin up a fresh queue branch
-          TIMESTAMP=$(date +%Y%m%d-%H%M%S)
-          NEW_BRANCH="merge-queue/${MERGE_TRAIN_BRANCH}/${TIMESTAMP}"
-          git checkout -b "$NEW_BRANCH"
+          git fetch origin "$MERGE_TRAIN_BRANCH" || true
+          git checkout "$MERGE_TRAIN_BRANCH" || git checkout -b "$MERGE_TRAIN_BRANCH"
+          git reset --hard "$BASE_BRANCH"
 
-          # Merge the base branch (typically 'next') with theirs strategy
-          BASE_BRANCH="${MERGE_TRAIN_BRANCH#merge-train/}"
-          git merge -X theirs "origin/$BASE_BRANCH"
-
-          # Seed it with an empty commit
+	  # Start with an empty commit
           git commit --allow-empty -m "[empty] Start merge-train. Choo choo."
 
           # Push & open the PR
@@ -54,6 +51,4 @@ jobs:
             --base "$MERGE_TRAIN_BRANCH" \
             --head "$NEW_BRANCH" \
             --title "feat: $MERGE_TRAIN_BRANCH" \
-            --body "This is an automated merge-queue PR. New commits from \`next\` will be added here.
-
-            When ready, this PR will be merged automatically."
+            --body "This is an automated merge-train PR with no commits."