ADR 0006: Adopt existing repositories conservatively

DocKit-FP discovers only a root README.md and Markdown inside docs/ when initialising an existing repository. This provides a useful first site without accidentally publishing release notes, contribution policies, security guidance or arbitrary private Markdown. It records ancillary root files only as available options.

The generated layout.json is the boundary between convention and control. It is created once and then treated as normal maintainer-owned configuration; no later discovery changes navigation.

The root README uses an explicit "source": "root" marker which accepts only the exact README.md. This avoids copied content and parent-path traversal, while repository-relative source resolution keeps links and immutable historical builds correct.