Add a Claude Code 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.

.claude/tasks/0008-claude-code-module.md

@@ -0,0 +1,22 @@
+---
+spec: laptop-mvi
+blocked-by: 0001-skeleton-and-building-host
+---
+
+## What to build
+
+Install Claude Code declaratively on `neogaia`, and make it authenticatable without a browser on the laptop so it can be used over the console/SSH via the paste-code flow or an API key.
+
+## Acceptance criteria
+
+- [x] Claude Code is installed declaratively (following the `Enable convention` if expressed as a `Module`) and enabled on `neogaia`.
+- [x] The browserless authentication path (paste-code flow or API key) is documented so it works over console/SSH.
+- [x] The `neogaia` toplevel still builds with Claude Code included.
+
+## Implementation Notes
+
+- **Native home-manager module, not a raw package.** Claude Code is enabled through home-manager's own `programs.claude-code` module (`home-manager.users.<user>.programs.claude-code.enable = true`), mirroring how `tmux`/`fish` use their native home-manager options rather than dropping a package into `home.packages`. The module ships within home-manager itself, so — unlike `nvim`/nixvim — no new flake input is needed. Per the invocation's steer to prefer the tmux/nvim conventions over the task wording, the feature `Module` at `modules/claude-code/claude-code.nix` is kept as thin as the `tmux` module: just the `enable` option and the delegation.
+- **No settings written.** The module manages no `~/.claude` contents and writes no `settings.json`, so login and first-run configuration stay interactive. This keeps auth material (subscription token or API key) out of the repo.
+- **Auth docs co-located with the module.** The browserless authentication guide lives at `modules/claude-code/authentication.md`, next to the module, following the repo pattern where each module directory holds its own supporting files. It covers both the paste-code OAuth flow (open the printed URL on another device, paste the code back — works unchanged over SSH) and the `ANTHROPIC_API_KEY` path. This is distinct from task 0009's OS-install docs, which cover `disko-install`, not the CLI login.
+- **Verification.** `nix build .#checks.x86_64-linux.neogaia` (the primary Host seam) builds the toplevel with `claude-code-2.1.209` included; `config.modules.claude-code.enable` and the home-manager `programs.claude-code.enable` both evaluate `true`.
+- **Note on flake evaluation.** The new module file had to be `git add`ed before the flake could see it — flakes evaluate the git tree, so an untracked Module is invisible to the Auto-loader and the host errors with "option does not exist".

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";

modules/claude-code/authentication.md

@@ -0,0 +1,32 @@
+# Authenticating Claude Code without a browser
+
+`neogaia` is driven from the console and over SSH, where no local browser can service Claude Code's default OAuth redirect.
+Either of the two paths below signs the CLI in from a bare terminal.
+Both are one-time actions per machine; the credentials land under `~/.claude`, which home-manager does not overwrite.
+
+## Paste-code flow (Claude subscription or Console OAuth)
+
+Run `claude` and start the login with the `/login` command (the first run offers it automatically).
+On a machine with no browser it cannot open the authorization page itself, so it prints the authorization URL and waits.
+
+1. Copy the printed URL to a browser on any other device (phone, another laptop).
+2. Sign in and approve the request there.
+3. The page returns a short authorization code; paste it back at the `claude` prompt still waiting in the terminal.
+
+The session then completes and the token is stored, so later runs need no further login.
+Because the URL is opened on a *different* device, this works unchanged over SSH.
+
+## API key
+
+For non-interactive use, set an Anthropic API key from <https://console.anthropic.com> in the environment before launching `claude`:
+
+```console
+$ export ANTHROPIC_API_KEY=sk-ant-...
+$ claude
+```
+
+Claude Code reads `ANTHROPIC_API_KEY` on startup and skips the interactive login entirely, so this path needs neither a browser nor the paste-code exchange.
+Usage is billed to the Console account that owns the key rather than to a Claude subscription.
+
+The key is a secret and is deliberately not baked into this configuration.
+Export it from the shell for a one-off, or source it from a secret store once one exists on the Host.

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;
+  };
+}