fix(ci): address Copilot review — workflow_call, branch collision, merge failure [skip-regression-check]

1. Add workflow_call trigger to test.yml and e2e.yml so staging-ci
   can invoke them as reusable workflows.

2. Add github.run_id suffix to promotion branch name to prevent
   collision on workflow reruns.

3. Merge step now fails hard instead of swallowing errors. If merge
   fails, the step fails and the PR stays open.

4. update-tag intentionally advances regardless of gate outcome —
   this is by design so the next batch covers only new commits.
   Blocked batches stay as open PRs with review comments.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

.github/workflows/e2e.yml

@@ -1,5 +1,6 @@
 name: E2E Tests
 on:
+  workflow_call:
   schedule:
     - cron: "0 6 * * 1"  # Weekly Monday 6 AM UTC
   workflow_dispatch:

.github/workflows/staging-ci.yml

@@ -146,7 +146,7 @@ jobs:
         if: steps.ahead-check.outputs.commits_ahead != '0'
         run: |
           SHORT_SHA=$(echo "${{ needs.check-changes.outputs.current_head }}" | cut -c1-8)
-          BRANCH="staging-promote/${SHORT_SHA}"
+          BRANCH="staging-promote/${SHORT_SHA}-${{ github.run_id }}"
           git checkout -b "$BRANCH"
           git push origin "$BRANCH"
           echo "branch=${BRANCH}" >> "$GITHUB_OUTPUT"
@@ -392,6 +392,7 @@ jobs:
           fi
 
       - name: Merge promotion PR
+        id: merge
         if: steps.evaluate.outputs.passed == 'true'
         env:
           GH_TOKEN: ${{ steps.token.outputs.token }}
@@ -399,7 +400,8 @@ jobs:
         run: |
           if [ -n "$PR_NUMBER" ]; then
             echo "Merging promotion PR #${PR_NUMBER}"
-            gh pr merge "$PR_NUMBER" --merge --delete-branch || echo "::warning::Auto-merge failed for PR #${PR_NUMBER}"
+            gh pr merge "$PR_NUMBER" --merge --delete-branch
+            echo "merged=true" >> "$GITHUB_OUTPUT"
           fi
 
   # ── Update tested tag (always, so next batch covers only new commits) ──

.github/workflows/test.yml

@@ -1,5 +1,6 @@
 name: Run Tests
 on:
+  workflow_call:
   pull_request:
   push:
     branches: