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

hosts/hades/default.nix

@@ -20,12 +20,13 @@
   my = {
     gaming.enable = true;
     ai.local.enable = true;
+    pi.enable = true;
   };
 
   # ---------------------------------------------------------------------------
   # Host display configuration (absorbed by home/hypr/hyprland.nix)
   # ---------------------------------------------------------------------------
-  home-manager.users.${config.my.username}.volt.monitors = [
+  home-manager.users.${config.my.username}.hypr.monitors = [
     {
       # Samsung Odyssey Ark - matched by description so it survives port changes
       output = "desc:Samsung Electric Company Odyssey Ark H1AK500000";