Add ads.txt for AdSense apex-domain review

Needed once codeuctivity.cloud becomes the site's real GitHub Pages
custom domain: ad-network crawlers check ads.txt at the apex/root
domain, mirroring the existing app-ads.txt publisher line.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01McTLyKWfJF7H785ZaxEGom

Author: stesee

static/ads.txt

@@ -0,0 +1 @@
+google.com, pub-4008197181381361, DIRECT, f08c47fec0942fa0