home: centralize theme and make home-manager feature-aware

- add home/theme.nix as the single Silicon Volt palette and pass it to
  home modules via extraSpecialArgs
- drive foot, hyprland, hyprlock, mako, fzf, starship, rofi and the
  herdr picker from that palette; raw config files use @color@ tokens
  substituted at build time
- pass my = config.my into home-manager; add my.pi.enable, my.flakeDir
- gate home/pi.nix on my.pi.enable (enabled on hades)
- route zshconf/hyprconf aliases and FLAKE_CONFIG through my.flakeDir
- rename the hyprland monitor option volt.monitors -> hypr.monitors
- fix use_auto_tmux command-substitution bug in zsh/init.zsh
- track home/nvim/config/lazy-lock.json for reproducible plugins
- document theming and toggles in the README

home/nvim/default.nix

@@ -72,7 +72,11 @@ in
     ];
   };
 
-  # Symlink LazyVim configuration to ~/.config/nvim recursively
+  # Symlink LazyVim configuration to ~/.config/nvim recursively. This includes
+  # lazy-lock.json, which pins every plugin to a specific commit for
+  # reproducible builds. To update plugins, run `:Lazy update` in a writable
+  # copy and commit the resulting lazy-lock.json back to this repo, since the
+  # linked file is read-only.
   xdg.configFile."nvim" = {
     source = ./config;
     recursive = true;