docs(readme): convert README.adoc -> Markdown README.md (#25)

README must be real Markdown to render in GitHub community-health, the GitHub
profile, and external MCP directories (Glama) — AsciiDoc shows as raw markup
there. pandoc asciidoc->GFM, badges fixed to clickable, SPDX header kept as an
HTML comment, duplicate README.adoc removed.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: hyperpolymath