feat: extend Renovate config from org-level preset

Replaces the duplicated Renovate config with a simple extend from the
org-level preset (privilegedescalation/.github:renovate-config). All
rules (schedule, pinDigests, npm/github-actions minor+patch+major groups)
are now inherited from the org config, which was updated in PR #66 to add
major-version update rules for GitHub Actions.

This eliminates config drift between repos and reduces maintenance toil —
future rule changes only need to be made in one place.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

renovate.json

@@ -1,21 +1,5 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
-  "extends": ["config:recommended"],
-  "baseBranches": ["main"],
-  "schedule": ["every weekend"],
-  "prConcurrentLimit": 10,
-  "pinDigests": true,
-  "packageRules": [
-    {
-      "matchManagers": ["npm"],
-      "matchUpdateTypes": ["minor", "patch"],
-      "groupName": "npm minor and patch"
-    },
-    {
-      "matchManagers": ["github-actions"],
-      "matchUpdateTypes": ["minor", "patch"],
-      "groupName": "github-actions minor and patch"
-    }
-  ]
+  "extends": ["github>privilegedescalation/.github:renovate-config"]
 }