Add cooldown to dependabot uv config

Add a 4-day cooldown for uv dependency updates as recommended by the
uv dependabot integration docs, to avoid PRs that uv cannot resolve
due to exclude-newer constraints.

Author: jonathangreen

.github/dependabot.yml

@@ -8,6 +8,8 @@ updates:
     schedule:
       interval: "daily"
     versioning-strategy: increase-if-necessary
+    cooldown:
+      default-days: 4
 
   - package-ecosystem: "github-actions"
     directory: "/"