Serve /evaluate/prompt.md and /agent-setup/prompt.md as text/plain: ChatGPT's web reader refuses text/markdown

定位提供 /evaluate/prompt.md 和 /agent-setup/prompt.md 的 hosting 或 header 配置；该 issue 引用了现有的 llms.txt 和 domain-bundle 行为，以及 nextcommerce-site#176 中对应的 Netlify 规则。更新这两个路径，使其返回 text/plain; charset=UTF-8，然后运行所述的 curl -sI 检查，并确认路径和引用保持不变。

Gap: both agent prompt files are served with Content-Type: text/markdown. ChatGPT's web reader refuses to open that type. Observed twice on 2026-09-16 against nextcommerce.com/agent-eval/prompt.md (same content type, same behaviour): "the web reader here is rejecting that endpoint because it serves text/markdown directly". The URL was confirmed 200 to ChatGPT-User, OAI-SearchBot and GPTBot user agents, so it is the type, not access.

Codex CLI, Claude Code and Cursor fetch these fine (plain HTTP clients), which is why the persona × agent probes never hit it. ChatGPT web is the surface a prospect on the marketing site reaches through the hero strip, and the www consultation file hands the developer persona to /evaluate/prompt.md, so a ChatGPT user gets one hop and then a wall.

Fix: serve /evaluate/prompt.md and /agent-setup/prompt.md as text/plain; charset=UTF-8, as llms.txt and the domain bundles already are. Paths unchanged; they are referenced from llms.txt and from the www file. nextcommerce-site did the same in NextCommerceCo/nextcommerce-site#176 with a Netlify header rule.

Verify: curl -sI https://developers.nextcommerce.com/evaluate/prompt.md | grep -i content-type.