chore: add v4-next branch automation

.github/README.md

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+# v4-next Branch
+
+`v4-next` is a forward-looking branch that builds on top of `v4`. It contains everything in `v4` plus additional features and changes that are staged for the next v4 release cycle.
+
+## How it works
+
+### Branch relationship
+
+```
+next (v5 development)
+  │
+  v4  ──────────────►  v4-next
+  (stable v4 releases)   (next v4 features)
+```
+
+- `v4-next` is branched off `v4` and stays up to date with it automatically.
+- Changes merged to `v4` flow into `v4-next` on every merge via the `pull-v4-into-v4-next` workflow.
+- If a merge conflict occurs, a PR is automatically created for manual resolution.
+
+### Getting changes into v4-next
+
+There are three ways to land code in `v4-next`:
+
+1. **Via v4**: Any PR merged to `v4` is automatically pulled into `v4-next`.
+2. **Backport from next**: Add the `backport-to-v4-next` label to a PR targeting `next`. On merge, the backport workflow will cherry-pick the changes into `v4-next` via a staging branch.
+3. **Direct PR**: Open a PR directly targeting `v4-next` for changes that are only relevant to the next release.
+
+### Nightly releases
+
+Every night at 5:00 AM UTC, `nightly-release-tag-v4-next.yml` creates a tag from `v4-next` in the format `v{version}-nightly.{date}`. This mirrors the nightly release process on `next`.
+
+### Backports
+
+The existing backport infrastructure (`backport.yml`) works with `v4-next` out of the box:
+
+- Label a PR with `backport-to-v4-next` to backport it.
+- On merge, the changes are cherry-picked onto a `backport-to-v4-next-staging` branch.
+- If cherry-pick fails, ClaudeBox is dispatched to resolve conflicts automatically.
+- A staging PR accumulates backported commits targeting `v4-next`.
+
+### Automation summary
+
+| Workflow | Trigger | Action |
+|---|---|---|
+| `pull-v4-into-v4-next.yml` | Push to `v4` | Merges `v4` into `v4-next`; creates conflict PR if needed |
+| `nightly-release-tag-v4-next.yml` | Daily at 05:00 UTC | Tags `v4-next` with `v{version}-nightly.{date}` |
+| `backport.yml` | `backport-to-v4-next` label + PR merge | Cherry-picks into `v4-next` via staging branch |

.github/workflows/nightly-release-tag-v4-next.yml

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+name: Nightly Release Tag (v4-next)
+on:
+  schedule:
+    # Run the workflow every night at 5:00 AM UTC.
+    - cron: "0 5 * * *"
+  workflow_dispatch: {}
+
+permissions:
+  contents: write
+
+concurrency:
+  group: ${{ github.workflow }}-v4-next
+
+jobs:
+  nightly-release-tag-v4-next:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
+        with:
+          ref: v4-next
+          token: ${{ secrets.AZTEC_BOT_GITHUB_TOKEN }}
+
+      - name: Create v4-next Nightly Tag
+        run: |
+          git config --global user.email "tech@aztecprotocol.com"
+          git config --global user.name "AztecBot"
+          current_version=$(jq -r '."."' .release-please-manifest.json)
+          echo "Current version: $current_version"
+          nightly_tag="v${current_version}-nightly.$(date -u +%Y%m%d)"
+          echo "v4-next nightly tag: $nightly_tag"
+          git tag -a "$nightly_tag" -m "$nightly_tag"
+          git push origin "$nightly_tag"

.github/workflows/pull-v4-into-v4-next.yml

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+name: Pull v4 into v4-next
+
+on:
+  push:
+    branches:
+      - v4
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  pull-v4-into-v4-next:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout v4-next
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
+        with:
+          ref: v4-next
+          fetch-depth: 0
+          token: ${{ secrets.AZTEC_BOT_GITHUB_TOKEN }}
+
+      - name: Configure Git
+        run: |
+          git config user.name "AztecBot"
+          git config user.email "tech@aztecprotocol.com"
+
+      - name: Attempt to merge v4 into v4-next
+        id: merge
+        run: |
+          git fetch origin v4
+          if git merge origin/v4 --no-edit; then
+            echo "conflict=false" >> $GITHUB_OUTPUT
+            echo "Merge succeeded without conflicts."
+          else
+            git merge --abort
+            echo "conflict=true" >> $GITHUB_OUTPUT
+            echo "Merge has conflicts."
+          fi
+
+      - name: Push merged v4-next
+        if: steps.merge.outputs.conflict == 'false'
+        run: git push origin v4-next
+
+      - name: Create conflict-resolution PR
+        if: steps.merge.outputs.conflict == 'true'
+        id: conflict-pr
+        env:
+          GH_TOKEN: ${{ secrets.AZTEC_BOT_GITHUB_TOKEN }}
+        run: |
+          SYNC_BRANCH="sync/v4-into-v4-next-$(date -u +%Y%m%d-%H%M%S)"
+          git merge --abort 2>/dev/null || true
+          git checkout -b "$SYNC_BRANCH"
+
+          # Merge with conflicts left in place
+          git merge origin/v4 --no-commit --no-ff || true
+          git add -A
+          git commit -m "chore: merge v4 into v4-next (conflicts need resolution)" || true
+          git push origin "$SYNC_BRANCH"
+
+          # Check if there's already an open sync PR by branch naming convention
+          EXISTING_PR=$(gh pr list --base v4-next --state open --json number,headRefName --jq '[.[] | select(.headRefName | startswith("sync/v4-into-v4-next-"))][0].number // empty')
+
+          if [ -n "$EXISTING_PR" ]; then
+            gh pr comment "$EXISTING_PR" --body "New v4 changes pushed. A fresh sync branch \`$SYNC_BRANCH\` has been created. Please resolve conflicts there or close this PR and use the new branch."
+            echo "pr_url=$(gh pr view "$EXISTING_PR" --json url --jq '.url')" >> $GITHUB_OUTPUT
+          else
+            PR_URL=$(gh pr create \
+              --base v4-next \
+              --head "$SYNC_BRANCH" \
+              --title "chore: merge v4 into v4-next (resolve conflicts)" \
+              --body "$(cat <<'EOF'
+## Automatic v4 → v4-next sync
+
+This PR was created because merging `v4` into `v4-next` resulted in conflicts.
+
+Please resolve the conflicts and merge this PR to keep `v4-next` up to date.
+
+This PR was auto-generated by the `pull-v4-into-v4-next` workflow.
+EOF
+              )")
+            echo "pr_url=$PR_URL" >> $GITHUB_OUTPUT
+          fi
+
+      - name: Notify Slack
+        if: steps.merge.outputs.conflict == 'true'
+        env:
+          SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
+        run: |
+          PR_URL="${{ steps.conflict-pr.outputs.pr_url }}"
+          TEXT=$(printf '⚠️ Auto-merge v4 → v4-next failed due to conflicts. <%s|View PR>. @Claudebox' "$PR_URL")
+          curl -sS -X POST https://slack.com/api/chat.postMessage \
+            -H "Authorization: Bearer $SLACK_BOT_TOKEN" \
+            -H "Content-type: application/json" \
+            -d "$(jq -n --arg c "#backports" --arg t "$TEXT" '{channel:$c, text:$t}')"