codeSplitters: verify Cut's hosted test run on Cloudflare

Follow-up to #193. The agent sandbox image now installs bun and its server answers POST /test (cut runs), tested locally only. It hasn't been built or run on Cloudflare: deploy with CODESPLITTERS_AGENTS=1, run a cut as an admin, and check the gutter marks and that the container stops afterwards. Also check that X unfurls a live /api/cuts/:id/share link with its card.