feat: add Renovate config to keep Dockerfile updated (#38)

Add renovate.json with a custom regex manager that picks up
`# renovate:` comment directives on ARG lines. This covers:
- Docker base images (handled natively by Renovate's dockerfile manager)
- git-delta, fzf, and zsh-in-docker versions via github-releases

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

renovate.json

@@ -0,0 +1,25 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": ["config:recommended"],
+  "schedule": ["before 9am on monday"],
+  "minimumReleaseAge": "7 days",
+  "packageRules": [
+    {
+      "description": "Group all updates into a single PR",
+      "matchPackageNames": ["*"],
+      "groupName": "all dependencies",
+      "groupSlug": "all"
+    }
+  ],
+  "customManagers": [
+    {
+      "customType": "regex",
+      "description": "Update ARG versions in Dockerfile from GitHub releases",
+      "fileMatch": ["(^|/)Dockerfile$"],
+      "matchStrings": [
+        "# renovate: datasource=(?<datasource>[a-z-]+?) depName=(?<depName>.+?)\\n(?:ENV|ARG) .+?_VERSION=(?<currentValue>.+?)\\n"
+      ],
+      "extractVersionTemplate": "^v?(?<version>.+)$"
+    }
+  ]
+}