chore(nixos): update Pi default model

AGENTS.md

@@ -120,3 +120,4 @@
   Do not use `git subtree add` or `git subtree pull` for Pi PR branches.
   Gitea rebases the synthetic upstream root commit without the `third_party/pi` prefix.
   Preserve `git-subtree-dir` and `git-subtree-split` trailers on Pi vendor commits.
+- The default Pi model for NixOS user configuration is set in `configs/nixos/modules/shell/pi/default.nix`, not in vendored Pi source.

configs/nixos/modules/shell/pi/default.nix

@@ -29,7 +29,7 @@ in
 
         settings = {
           defaultProvider = "openai-codex";
-          defaultModel = "gpt-5.5";
+          defaultModel = "gpt-5.6-terra";
           defaultThinkingLevel = "medium";
           theme = "dark";
           enableInstallTelemetry = false;