fstack is a dual-engine skill stack for founders who still write the product. One router classifies the request: founder operations (inbound leads, support, social, strategy, competitive teardown) or engineering (/engineer-mode with 23 playbooks and 24 principles). The bridge skill is /support-loop: diagnose the customer issue, fix the bug with a failing test, draft the reply you can send. Install once. Same skills in ChatGPT, Codex, Cursor, and Claude Code.