Replace dependabot with renovate for GitHub Actions updates

Co-Authored-By: Claude (Opus 4.6) <noreply@anthropic.com>

.github/dependabot.yml

@@ -1,10 +0,0 @@
-version: 2
-
-updates:
-  - package-ecosystem: github-actions
-    labels: [modifies/dependencies]
-    directory: /
-    schedule:
-      interval: daily
-    cooldown:
-      default-days: 5

renovate.json

@@ -0,0 +1,8 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": ["config:recommended"],
+  "enabledManagers": ["github-actions"],
+  "labels": ["modifies/dependencies"],
+  "schedule": ["before 6am"],
+  "minimumReleaseAge": "5 days"
+}