ci: guard against undefined PRAUTOBLOGGER_* constants (#165)

Adds scripts/check-constants.php + a required ci.yml step that fails the build when any referenced PRAUTOBLOGGER_* constant is neither defined in prautoblogger.php nor defined()-guarded. Closes the v0.19.0 admin-500 incident class (seq 38). CI-only; no plugin runtime change.

QA: APPROVE-WITH-P2-NOTES (2026-06-14-923f712). P2 follow-ups folded into M4.

Agent-Session: ci-constants-guard-20260614

.github/workflows/ci.yml

@@ -109,3 +109,20 @@ jobs:
         run: composer install --no-progress --no-interaction --prefer-dist
       - name: Run PHPUnit (Brain Monkey — no WP install required)
         run: vendor/bin/phpunit --testdox
+
+  check-constants:
+    name: Check PRAUTOBLOGGER_* constants
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Setup PHP 8.1
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.1'
+          coverage: none
+      - name: Check PRAUTOBLOGGER_* constants
+        # Fails if any PRAUTOBLOGGER_* constant referenced in the shipped
+        # plugin PHP is neither defined in prautoblogger.php nor guarded
+        # by defined() at the reference site. Prevents the v0.19.0 incident
+        # class (constant deleted from bootstrap, test fallback masks the gap).
+        run: php scripts/check-constants.php