ci: add PRAUTOBLOGGER_* constants guard (scripts/check-constants.php)

Wires a new required CI step — 'Check PRAUTOBLOGGER_* constants' — that
fails the build when any PRAUTOBLOGGER_* constant referenced in the
shipped plugin PHP is neither:
  (a) define()d in prautoblogger.php (including conditional OPIK defines), nor
  (b) protected by a defined() guard at the reference site.

This closes the regression class that caused the v0.19.0 admin-500 incident:
PR #156 deleted three constants from prautoblogger.php; CI stayed green because
tests/bootstrap.php holds fallback copies, masking the gap until prod boot.

Deliverables:
  - scripts/check-constants.php (212 lines, no Composer deps, PHP 8.1+)
  - .github/workflows/ci.yml: new 'check-constants' job
  - CHANGELOG.md: one-line note under [Unreleased - Internal]

No plugin version bump (CI-only; shipped artifact unchanged).

AC-1 verified locally (Python3 equivalent logic):
  - HEAD passes clean (0 violations)
  - Remove PRAUTOBLOGGER_DEFAULT_RUN_CEILING_USD -> 2 violations caught
  - Remove PRAUTOBLOGGER_DEFAULT_REQUEST_JSON_RETENTION_DAYS -> 3 violations caught
  - Remove PRAUTOBLOGGER_DEFAULT_REASONING_MAX_TOKENS -> 1 violation caught

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