Add a 30-day Dependabot cooldown (#400)

The repository has no explicit Dependabot cooldown, so routine updates
do not follow the 30-day policy used by `setup-uv`. Add
`cooldown.default-days: 30` for npm and GitHub Actions while retaining
the daily schedule. Dependabot security updates remain outside the
cooldown.

Co-authored-by: zaniebot <242828183+zaniebot@users.noreply.github.com>

.github/dependabot.yml

@@ -4,8 +4,12 @@ updates:
     directory: /
     schedule:
       interval: daily
+    cooldown:
+      default-days: 30
 
   - package-ecosystem: npm
     directory: /
     schedule:
       interval: daily
+    cooldown:
+      default-days: 30