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.

hosts/neogaia/default.nix

@@ -50,6 +50,7 @@
   modules.nvim.enable = true;
   modules.claude-code.enable = true;
   modules.gitea-axi.enable = true;
+  modules.pi.enable = true;
 
   time.timeZone = "America/New_York";
   i18n.defaultLocale = "en_GB.UTF-8";