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.
- Remove duplicate CHANGELOG.md (CHANGELOG.adoc is canonical).

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

Author: claude

CHANGELOG.md

@@ -7,6 +7,8 @@ Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 :icons: font
 :source-highlighter: rouge
 
+image:https://img.shields.io/badge/Sponsor-%E2%9D%A4-pink?logo=github[Sponsor,link=https://github.com/sponsors/hyperpolymath]
+
 == What Is a2mliser?
 
 a2mliser wraps any markup, configuration, or manifest file in an