fix: add Cargo monitoring to Dependabot config

Had only github-actions monitored. Now also scans Cargo.lock
weekly with minor+patch grouping.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/dependabot.yml

@@ -8,3 +8,11 @@ updates:
       actions:
         patterns:
           - "*"
+
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    groups:
+      rust-minor:
+        update-types: ["minor", "patch"]