docs: de-dup README/CHANGELOG per RSR standard (keep .adoc; port sponsor badge)

Brings this repo in line with the family-wide doc de-dup that landed across
the -iser repos during base camp but was missed here (this repo was
squash-merged before the cleanup).

- Remove stale README.md (README.adoc is canonical), porting the GitHub
  sponsor badge into README.adoc.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Mbq6yKF9RhFai6EQ7WqKhQ

Author: claude