chore: drop legacy Makefiles + recap justfile -> Justfile

asdf-augmenters/asdf-ghjk/Makefile and asdf-ghjk/Makefile removed
in favour of the Justfile-based task runner. asdf-augmenters/
asdf-plugin-collection's lowercase justfile renamed to Justfile
(estate-wide capitalization).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath