feat(claude-code): add a Module and enable it on neogaia

Install Claude Code through home-manager's native programs.claude-code
module, matching how the tmux and fish Modules use their home-manager
options rather than a raw home.packages entry. The feature Module at
modules/claude-code/claude-code.nix stays thin — just the enable option
and the delegation — and writes no settings, so login and first-run
configuration remain interactive and no auth material enters the repo.

Signing in without a browser (needed over the console or SSH) is
documented in modules/claude-code/authentication.md: the paste-code OAuth
flow, where the printed URL is opened on another device and the code
pasted back, and the ANTHROPIC_API_KEY path for non-interactive use.

The neogaia toplevel builds with claude-code-2.1.209 included.

hosts/neogaia/default.nix

@@ -45,6 +45,9 @@
   # Neovim, configured declaratively via nixvim.
   modules.nvim.enable = true;
 
+  # Claude Code, Anthropic's CLI, installed via home-manager.
+  modules.claude-code.enable = true;
+
   # Locale preferences for the base system.
   time.timeZone = "America/New_York";
   i18n.defaultLocale = "en_GB.UTF-8";