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

README.md

@@ -36,13 +36,14 @@ my = {
   username = "anish";      # default; change for a different primary user
   gaming.enable = false;   # Steam, Gamemode, MangoHud
   ai.local.enable = false; # Ollama, ComfyUI, Open WebUI
+  pi.enable = false;       # pi coding agent (home-manager)
 };
 ```
 
 4. Declare host-specific displays in `hosts/<name>/default.nix`:
 
 ```nix
-home-manager.users.anish.volt.monitors = [
+home-manager.users.anish.hypr.monitors = [
   {
     output = "eDP-1";              # or "desc:<panel description>"
     mode = "2880x1800@120Hz";      # or "preferred"
@@ -54,7 +55,7 @@ home-manager.users.anish.volt.monitors = [
 ];
 ```
 
-The `volt.monitors` default (any output @ preferred) keeps every machine working
+The `hypr.monitors` default (any output @ preferred) keeps every machine working
 out of the box; hosts override only what they need. Hosts without an NVIDIA GPU
 simply don't import `modules/hardware/nvidia.nix` — all vendor env vars live there,
 while the vendor-neutral `hardware.graphics` stack lives in `modules/graphical`.
@@ -68,7 +69,17 @@ import lists:
 | Option | Enables |
 | --- | --- |
 | `my.username` | Primary user (users, home-manager, getty autologin) |
+| `my.flakeDir` | Checkout path used by the `rebuild` helper and `zshconf`/`hyprconf` aliases |
 | `my.gaming.enable` | Steam, Gamemode, MangoHud |
 | `my.ai.local.enable` | Ollama (CUDA), ComfyUI, Open WebUI |
+| `my.pi.enable` | pi coding agent (home-manager) |
 
 Hardware modules remain explicit imports per host (e.g. `modules/hardware/nvidia.nix`).
+
+## Theming
+
+The Silicon Volt palette lives in one place, `home/theme.nix`, and is passed to
+every home-manager module through `extraSpecialArgs`. Tools that only accept a
+raw config file (starship, rofi, the herdr picker) carry `@color@` placeholders
+that are substituted from the same palette at build time, so changing an accent
+means editing one file.