Add gen-update-targets.sh to parse flake.nix and generate grouped
update targets (update-kyle, update-mc). Makefile now has install
(copy) and link (symlink) targets for rebuild-nixos. Also fix mc
flake input URLs to use /mc/ org path.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
