refactor(nix): deployment in vm.nix only; host uses chezmoi for dotfiles

Per user decision: on the Arch host, chezmoi remains the single deployer
of $HOME dotfiles. nix/common.nix's xdg.configFile + sshConfig
activation + .zshenv home.file block was causing home-manager to fight
chezmoi on every nix-switch, materializing .backup files for nvim,
zellij, zsh, git, ghostty, direnv.

Resolution:

- nix/common.nix: drop the entire deployment block, drop the
  my.dotfilesPath option, drop the let..in dotfiles/link helpers.
  Module is now deployment-agnostic: only installs packages.

- nix/host.nix: drop my.dotfilesPath; explicit comment that chezmoi
  owns dotfile deployment on the host.

- nix/vm.nix: gains everything previously in common.nix's deployment
  block — xdg.configFile (nvim/zellij/zsh/git/ghostty/direnv),
  home.activation.sshConfig, home.file.".zshenv". The 'dotfiles'
  let-binding (= $HOME/.local/share/dotfiles) and 'link' helper move
  here too, since they're vm-only now.

Host runbook unchanged (`just sync`); first run after pulling will
just be a no-op nix-switch instead of a backup-file storm.

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