docs: write public-facing reference docs in docs/ #3

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.