chore: add dependabot for weekly dependency updates (#208)

## Summary
- Adds Dependabot configuration for Cargo dependencies
- Weekly schedule with all dependencies grouped into single PR

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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: shikhar

.github/dependabot.yml

@@ -0,0 +1,10 @@
+version: 2
+updates:
+  - package-ecosystem: cargo
+    directory: /
+    schedule:
+      interval: weekly
+    groups:
+      all-dependencies:
+        patterns:
+          - "*"