Group Dependabot version updates into a single weekly PR per ecosystem

Adds a "groups" block with patterns ["*"] to the github-actions ecosystem
so all bumps land in one weekly PR, and applies the "autoupdate" label.

Also adds a pre-commit Dependabot ecosystem entry (grouped, labeled) and
deletes the now-redundant .github/workflows/pre-commit-autoupdate.yml,
whose flow is now handled by Dependabot.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: antonwolfy

.github/dependabot.yml

@@ -4,3 +4,16 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    labels: ["autoupdate"]
+    groups:
+      github-actions:
+        patterns: ["*"]
+
+  - package-ecosystem: "pre-commit"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    labels: ["autoupdate"]
+    groups:
+      pre-commit:
+        patterns: ["*"]