feat(pi): install the coding agent for a side-by-side trial (task 0020)

Add a `pi` module that turns on the home-manager `programs.pi-coding-agent`
integration for the primary user and freezes a single settings.json: the
default provider set to Anthropic, the default model to Opus (claude-opus-4-8,
Pi's own catalogue id), and analytics disabled. Everything else in
~/.pi/agent — the credential and all self-modification state — is left
unmanaged, so login survives rebuilds and backing Pi out is a one-line enable
flip. Enabled on neogaia alone.

modules/pi/pi.nix

@@ -11,7 +11,8 @@ let
   user = config.user.name;
 in
 {
-  options.modules.pi.enable = lib.mkEnableOption "Pi, a terminal coding agent, configured via home-manager";
+  options.modules.pi.enable = lib.mkEnableOption ''
+    Pi, a terminal coding agent, configured via home-manager'';
 
   config = lib.mkIf cfg.enable {
     home-manager.users.${user}.programs.pi-coding-agent = {