add pi coding agent module

Manages `~/.pi/agent` config via the itme-brain/pi submodule and
provides nodejs_20 with NPM_CONFIG_PREFIX=~/.npm-global so pi and its
extensions install via the global npm ecosystem.

Pi itself and extensions like pi-mcp-adapter are installed manually
via `npm install -g` rather than packaged through nix — the ecosystem
moves too fast to keep derivations current.

system/machines/desktop/modules/home-manager/home.nix

@@ -81,6 +81,8 @@
         writing.enable = true;
       };
 
+      pi.enable = true;
+
       gui = {
         wm.hyprland.enable = true;
         browser.firefox.enable = true;