Rewrite bootstrap, cut down Makefile, update README

Rewrite bootstrap, cut down Makefile, update README bootstrap.sh becomes four steps: Xcode CLI tools, Determinate Nix, sdkman, first switch. It takes an optional host argument so the work Mac uses the same entry point. sdkman is installed with rcupdate=false on purpose. .zshrc is now a symlink into this repo, and sdkman's installer would otherwise append its init block to the tracked file; .zshrc sources sdkman itself instead. The first switch runs darwin-rebuild straight from the flake, because the command does not exist yet on a fresh machine, and resolves nix by absolute path first, because sudo strips /nix/... from PATH. The Makefile drops from ~380 lines to four targets. install, require-chezmoi, bootstrap-tools, backup, clean, clean-tools, clean-all, brew-clean, brew-clean-force, brew-trust, defaults, dump, doctor-soft-clean, the 40-test check target, and all four lazyvim targets are gone: generations replace the backup and clean machinery, and nix flake check replaces the file tests. mas-update survives because homebrew.masApps installs but does not upgrade. brew upgrade moves into `make update` because onActivation.upgrade is off, keeping activation deterministic. chezmoi is no longer used by anything in this repo. The binary at ~/.local/bin/chezmoi predates this migration and is left alone.