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

modules/options.nix

@@ -1,4 +1,5 @@
 {
+  config,
   lib,
   ...
 }:
@@ -13,8 +14,16 @@
       description = "Primary interactive user account.";
     };
 
+    flakeDir = lib.mkOption {
+      type = lib.types.str;
+      default = "/home/${config.my.username}/NixOS-Configs";
+      description = "Checkout location of this flake on the machine.";
+    };
+
     gaming.enable = lib.mkEnableOption "Steam, Gamemode and gaming tooling";
 
     ai.local.enable = lib.mkEnableOption "local AI services (Ollama, ComfyUI, Open WebUI)";
+
+    pi.enable = lib.mkEnableOption "the pi coding agent (home-manager)";
   };
 }