revert(readme): restore canonical README.adoc (#51)

Restores canonical README.adoc and removes the README.md from the
over-broad 'convert -> Markdown' pass. AsciiDoc is canonical; .md only
on boj-server + hyperpolymath.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath