Add robots.txt with sitemap directive

Previously missing from S3, causing crawlers to see homepage HTML
instead of crawl directives. File lives in public/ so Vite copies
it to dist/ on build.

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

Author: gmoon

public/robots.txt

@@ -0,0 +1,4 @@
+User-agent: *
+Allow: /
+
+Sitemap: https://forkzero.ai/sitemap.xml