Fix Dependabot to use bun ecosystem instead of npm

Dependabot was updating package.json but not bun.lock because
the config declared npm as the package ecosystem. Switching to
the native bun ecosystem so lockfile updates work correctly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: joined

.github/dependabot.yml

@@ -1,6 +1,6 @@
 version: 2
 updates:
-    - package-ecosystem: npm
+    - package-ecosystem: bun
       directory: /
       schedule:
           interval: weekly