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.

@@ -0,0 +17,4 @@
+    Pins the provider to Anthropic and the model to Opus so Pi runs the same
+    model as Claude Code, and disables analytics. Authentication reuses an
+    existing Claude subscription and is deliberately left unmanaged, so no
+    credential enters the repo and re-auth survives rebuilds'';

@@ -0,0 +25,4 @@
+
+      # settings.json is the only file here Pi writes at runtime, and only its
+      # analytics keys. Disabling analytics removes that write, so freezing the
+      # file never fights Pi's self-modification, which targets other paths.

@@ -0,0 +17,4 @@
+    home-manager.users.${user}.programs.pi-coding-agent = {
+      enable = true;
+
+      settings = {

@@ -0,0 +25,4 @@
+      };
+    };
+  };
+}

@@ -0,0 +11,4 @@
+  user = config.user.name;
+in
+{
+  options.modules.pi.enable = lib.mkEnableOption "Pi, a terminal coding agent, configured via home-manager";

@@ -0,0 +11,4 @@
+  user = config.user.name;
+in
+{
+  options.modules.pi.enable = lib.mkEnableOption ''