Migrate dotfiles to Nix Flakes + home-manager + nix-darwin

[Copilot]

Homebrew/bash-script setup (Brewfile, install.sh) is mac-only, imperative, and hard to keep reproducible across machines. This introduces a Nix Flakes-based setup as the primary path, portable across macOS (nix-darwin) and Linux (NixOS/standalone home-manager), while leaving the legacy setup in place as fallback.

Flake entry point

• flake.nix: inputs (nixpkgs, nixpkgs-unstable, home-manager, nix-darwin) and outputs for darwinConfigurations.macbook, nixosConfigurations.linux-desktop, and standalone homeConfigurations."kevin@linux|darwin".

home-manager modules (shared across platforms)

• home/shells.nix — zsh, oh-my-zsh, zsh-completions, pure prompt
• home/editors.nix — vim/neovim, VSCode extensions, zed settings + AGENTS.md
• home/terminal.nix — tmux (+ tmux-powerline), ghostty
• home/programs.nix — CLI tools ported from Brewfile
• home/default.nix — wires the above together, sets home.stateVersion

System-level configs

• nix/darwin.nix — Caps Lock → Control remap, TouchID sudo, unfree packages allowed
• nix/linux.nix — minimal machine-agnostic NixOS base (placeholders for fileSystems/bootloader, meant to be overridden per host)

Docs

• README.md updated with Nix quickstart (macOS/Linux/standalone), repo layout, and machine-customization notes; existing Homebrew/bash flow kept as a documented "Legacy Homebrew setup" fallback.

Example — applying on a Mac:

git clone https://github.com/thiskevinwang/dotfiles.git && cd dotfiles
nix run nix-darwin -- switch --flake .#macbook

Validated end-to-end with nix flake check (no local nix binary in this environment, so run via a nixos/nix Docker container); this surfaced and required fixing several real issues: programs.vscode.profiles doesn't exist in the pinned home-manager release, pkgs.tmuxPlugins.tpm doesn't exist (switched to tmuxPlugins.tmux-powerline), webp renamed to libwebp, VSCode requiring nixpkgs.config.allowUnfree, and NixOS requiring fileSystems/bootloader assertions to satisfy evaluation.