chore: enable next-branch automation (Phase 2 flips) (#232)

- auto-backmerge.yml: enable the job (was if: false in Phase 1)
- pr-target-validator.yml: enforce instead of warn-only

These flips are the operational gate for the next-branch model. The
next branch and branch protection were created/applied before this PR.

.github/workflows/auto-backmerge.yml

@@ -27,7 +27,7 @@ permissions:
 jobs:
   backmerge:
     # Phase-1 gate: leave false until `next` exists. Flip to `true` in Phase 2.
-    if: false
+    if: true
     runs-on: ubuntu-latest
     timeout-minutes: 5
     steps:

.github/workflows/pr-target-validator.yml

@@ -26,7 +26,7 @@ jobs:
     timeout-minutes: 2
     env:
       # Phase-1: warn only. Phase-2: set to 'false' to enforce.
-      WARN_ONLY: 'true'
+      WARN_ONLY: 'false'
     steps:
       - name: Validate PR target branch
         uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0