docs: write public-facing reference docs in docs/

PRD.md and design/*.md are gitignored during incubation. Before tagging v0.1.0, publish them as docs/ (rename + unignore + audit for any leftover internal references that survived the public-flip cleanup commit `5aaa8e9`).

Files: PRD.md, design/resolved-config.md, design/runtime.md, design/features.md, design/compose.md, design/status.md.

Status doc may be retired or replaced with a proper roadmap once we go public.

阅读 PRD.md 和五个 design/*.md 文件，然后检查提交 5aaa8e9，以了解 public-flip 清理工作。将列出的材料发布到 docs/ 下，按需取消对源文件的忽略，审查是否有遗留的内部引用，并决定 status.md 是退役还是替换为 roadmap。

由索引模型根据 Issue 内容生成。