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.

modules/claude-code/claude-code.nix

@@ -0,0 +1,22 @@
+{
+  config,
+  lib,
+  ...
+}:
+# Claude Code — Anthropic's CLI — for the primary user, installed declaratively
+# through home-manager. home-manager ships the package and owns ~/.claude; no
+# settings are written here, so login and first-run configuration stay
+# interactive. Signing in without a browser, as needed over the console or SSH,
+# is covered in ./authentication.md.
+let
+  cfg = config.modules.claude-code;
+  user = config.user.name;
+in
+{
+  options.modules.claude-code.enable =
+    lib.mkEnableOption "Claude Code, Anthropic's CLI, installed via home-manager";
+
+  config = lib.mkIf cfg.enable {
+    home-manager.users.${user}.programs.claude-code.enable = true;
+  };
+}