feat(ci): merge-train/spartan-v6 (v5-next port) (#24517)

Hand-port of the v6 merge-train wiring directly onto `v5-next`, so a
`v6-next` forked off `v5-next` carries the automation immediately
(rather than waiting for `next` → `v5-next` propagation). Companion to
the `next`-based PR #24516.

Same three-file change as #24516:

- **`merge-train-create-pr.yml`** — generalize base-branch detection
from the hard-coded `*-v5` check to any `-v<N>` suffix (`-v5` →
`v5-next`, `-v6` → `v6-next`; v5 behavior unchanged). Add
`merge-train/spartan-v6` to the `ci-full-no-test-cache` list.
- **`merge-train-next-to-branches.yml`** — add `v6-next` to the push
trigger and a branch that auto-pulls `v6-next` into
`merge-train/spartan-v6`.
- **`merge-train-stale-check.yml`** — add a `spartan-v6` stale-check job
(`BASE_BRANCH: v6-next`, `#team-alpha`), matching this branch's
guard-free job style.

`private-port-next` stays scoped to `v5-next` bases (so `spartan-v6` PRs
are not auto-forward-ported), and `notify-private-sync` / nightly
release-tag / spartan-bench are untouched — `v6-next` is a public-first
staging line, not a release line.

### Branch sequence after this lands
1. Merge this PR into `v5-next`.
2. Create `v6-next` off `v5-next` (now carrying the v6 wiring).
3. Create `merge-train/spartan-v6` off `v6-next` — `create-pr` sees the
tip is already in `v6-next` and skips; the first real commit opens the
train PR against `v6-next`.

---
*Created by
[claudebox](https://claudebox.work/v2/sessions/706da64a13de6c13) ·
group: `slackbot`*

Author: ludamad

.github/workflows/merge-train-create-pr.yml

@@ -24,11 +24,11 @@ jobs:
             branch="${{ github.ref_name }}"
 
             # Determine base branch. Most trains target next; trains suffixed
-            # with -v5 (e.g. spartan-v5, fairies-v5) target the v5 release line
-            # (v5-next) instead.
+            # with -v<N> (e.g. spartan-v5, fairies-v5, spartan-v6) target the
+            # matching release line (v5-next, v6-next) instead.
             base_branch="next"
-            if [[ "$branch" == *-v5 ]]; then
-              base_branch="v5-next"
+            if [[ "$branch" =~ -v([0-9]+)$ ]]; then
+              base_branch="v${BASH_REMATCH[1]}-next"
             fi
 
             # Skip if this is a merge commit (check for multiple parents)
@@ -52,7 +52,7 @@ jobs:
 
               # Create PR with ci-no-squash label
               labels="ci-no-squash"
-              if [[ "$branch" == "merge-train/spartan" || "$branch" == "merge-train/spartan-v5" || "$branch" == "merge-train/ci" ]]; then
+              if [[ "$branch" == "merge-train/spartan" || "$branch" == "merge-train/spartan-v5" || "$branch" == "merge-train/spartan-v6" || "$branch" == "merge-train/ci" ]]; then
                 labels="$labels,ci-full-no-test-cache"
               fi
               # Trains targeting the v5 release line are ports into v5-next.

.github/workflows/merge-train-next-to-branches.yml

@@ -5,6 +5,7 @@ on:
     branches:
       - next
       - v5-next
+      - v6-next
 
 jobs:
   merge-to-trains:
@@ -27,13 +28,17 @@ jobs:
           GH_TOKEN: ${{ secrets.AZTEC_BOT_GITHUB_TOKEN }}
           COMMIT_SHA: ${{ github.sha }}
         run: |
-          # The -v5 trains track the v5 release line (v5-next); every other
-          # train tracks next. Sync only the trains fed by the branch that was
-          # just pushed.
+          # The -v<N> trains track the matching release line (v5-next, v6-next);
+          # every other train tracks next. Sync only the trains fed by the
+          # branch that was just pushed.
           if [[ "${{ github.ref_name }}" == "v5-next" ]]; then
             for branch in merge-train/spartan-v5 merge-train/fairies-v5; do
               ./scripts/merge-train/merge-next.sh "$branch" v5-next || true
             done
+          elif [[ "${{ github.ref_name }}" == "v6-next" ]]; then
+            for branch in merge-train/spartan-v6; do
+              ./scripts/merge-train/merge-next.sh "$branch" v6-next || true
+            done
           else
             for branch in merge-train/avm merge-train/barretenberg merge-train/ci merge-train/docs merge-train/fairies merge-train/spartan; do
               ./scripts/merge-train/merge-next.sh "$branch" || true

.github/workflows/merge-train-stale-check.yml

@@ -36,6 +36,21 @@ jobs:
           BASE_BRANCH: v5-next
         run: ./ci3/merge_train_stale_check merge-train/spartan-v5 '#team-alpha'
 
+  spartan-v6:
+    name: Check merge-train/spartan-v6
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: read
+    steps:
+      - uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5.0.1
+      - name: Run stale check
+        env:
+          GH_TOKEN: ${{ secrets.AZTEC_BOT_GITHUB_TOKEN }}
+          SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
+          BASE_BRANCH: v6-next
+        run: ./ci3/merge_train_stale_check merge-train/spartan-v6 '#team-alpha'
+
   fairies-v5:
     name: Check merge-train/fairies-v5
     runs-on: ubuntu-latest