revert(readme): restore canonical README.adoc; remove runaway .md conversion

The estate-wide 'convert README.adoc -> Markdown' pass over-applied the narrow
boj-server/hyperpolymath .md rule to every repo. AsciiDoc is canonical here;
restore README.adoc and remove README.md (clean revert).

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

Author: hyperpolymath