chore: add Dependabot config

Weekly Monday cadence matching fs-packages convention.

Composer ecosystem groups illuminate/* (synchronized Laravel releases)
and the phpunit/sebastian families (PHPUnit's own internal release
cycles). phpstan/phpstan deliberately ungrouped — every bump touches
the AST/scope API surface that the rules depend on, so each warrants
deliberate review.

GitHub Actions ecosystem also weekly Monday for workflow action bumps.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Goosterhof

.github/dependabot.yml

@@ -0,0 +1,24 @@
+version: 2
+
+updates:
+    - package-ecosystem: composer
+      directory: /
+      schedule:
+          interval: weekly
+          day: monday
+      groups:
+          illuminate:
+              patterns:
+                  - 'illuminate/*'
+          phpunit:
+              patterns:
+                  - 'phpunit/*'
+                  - 'sebastian/*'
+                  - 'phpunit/php-*'
+      open-pull-requests-limit: 10
+
+    - package-ecosystem: github-actions
+      directory: /
+      schedule:
+          interval: weekly
+          day: monday