Rework Makefile with install/link/update targets and auto-generation

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>

.gitignore

@@ -1,2 +1,3 @@
 /result
 /*-linux.tar.gz
+/update-targets.mk