[DEV-74] chore: add 30-day dependabot cooldown (#1041)

* chore: add 30-day dependabot cooldown

* Remove swift ecosystem from dependabot

Package.swift declares no external dependencies, so the swift
ecosystem entry is a no-op. Per review feedback from @tylerjroach.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Jin Yoo <jin.yoo@mixpanel.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 3 people

.github/dependabot.yml

@@ -0,0 +1,8 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    cooldown:
+      default-days: 30