feat(agents): make Pi a first-class agent

modules/agents/pi/pi.nix

@@ -0,0 +1,50 @@
+{
+  config,
+  lib,
+  ...
+}:
+# Pi, a terminal coding agent, for the primary user, configured through
+# home-manager, which ships the package and manages ~/.pi/agent.
+# The login credential is left unmanaged, so it survives rebuilds.
+let
+  cfg = config.modules.agents.pi;
+  user = config.user.name;
+  piDir = "${config.users.users.${user}.home}/.pi/agent";
+in
+{
+  options.modules.agents.pi.enable = lib.mkEnableOption ''
+    Pi, a terminal coding agent, configured via home-manager'';
+
+  config = lib.mkIf cfg.enable {
+    home-manager.users.${user} = {
+      programs.pi-coding-agent = {
+        enable = true;
+
+        settings = {
+          defaultProvider = "openai-codex";
+          defaultModel = "gpt-5.5";
+          defaultThinkingLevel = "medium";
+          theme = "dark";
+          enableInstallTelemetry = false;
+          enableAnalytics = false;
+        };
+      };
+
+      home.file = {
+        # The first declarative rollout replaces the interactive settings file.
+        # Login state stays in auth.json, which this module does not manage.
+        "${piDir}/settings.json".force = true;
+
+        "${piDir}/extensions" = {
+          source = ./extensions;
+          recursive = true;
+        };
+
+        "${piDir}/prompts" = {
+          source = ./prompts;
+          recursive = true;
+        };
+      };
+    };
+  };
+}