feat(git): enable the module per host rather than by default

Defaulting the module on hid it from a host's config, leaving no single
place that lists what a host carries. Each host enables it explicitly
instead, at the cost of a step when adding one.

.claude/tasks/0015-git-module.md

@@ -22,9 +22,9 @@ It is a `Module` rather than base plumbing because a `Host` that should not carr
 The module uses `settings.user.name`/`settings.user.email`.
 Do not "fix" it back.
 
-Review on the pull request asked for the module on every host, so `enable` defaults to true rather than being restated per host, and the explicit line in `hosts/neogaia/default.nix` is gone as redundant.
-Defaulting the option keeps the opt-out this task's rationale asked for, where moving the identity into the shared base config would not.
-The `neogaia` toplevel hashes identically before and after the change.
+Review on the pull request asked for the module on every host, which was first built by defaulting `enable` to true and dropping the per-host line.
+The operator then chose the opposite: `enable` defaults to false and each host enables it explicitly, so a host keeps reading as a full checklist of what it carries rather than hiding a default-on module.
+Enabling it is therefore a step when adding a host.
 
 The commit name is the literal `"alexion"` rather than `config.user.name`, which review raised as duplication.
 A Unix login and a commit display name are separate concepts that merely coincide here, so binding them would let a host overriding its login silently rewrite the operator's commit identity.

CLAUDE.md

@@ -36,8 +36,8 @@ The domain model (Host, Module, Skeleton, Auto-loader, Enable convention, overla
   Both were true only while the machine still ran CachyOS against a distro Nix daemon.
 - The substituters a `nix build` fetches from are the **daemon's** (`/etc/nix/nix.conf`), *not* the `nix.settings` of the config being built — those only govern the built system.
   The two coincide here because the dev host runs this flake; they diverge on any machine that does not.
-- Git identity is declared in the flake by `modules/git.nix`, which writes `alexion <contact@alexion.dev>` — the identity all history uses.
-  It is one of the few modules defaulting to on, so a host carries it without restating it and declines by setting `modules.git.enable = false`.
+- Git identity is declared in the flake by `modules/git.nix`, which writes `alexion <contact@alexion.dev>` — the identity all history uses — on any host enabling `modules.git`.
+  Every new host has to enable it, so that a host reads as a full checklist of what it carries.
   Once such a host has been rebuilt, a checkout on it needs no hand-written identity and keeps one across a reimage.
   Two things mask a broken module, so neither is evidence it works: this checkout's `.git/config` carries the same identity, and home-manager writes `~/.config/git/config` while a `~/.gitconfig` also exists and outranks it per key.
   That `~/.gitconfig` holds only a `tea` credential helper and no `user.*`, so it does not shadow the identity today, but it is undeclared and will not survive a reimage.

hosts/neogaia/default.nix

@@ -36,6 +36,7 @@
   modules.fish.enable = true;
   modules.fish.defaultShell = true;
 
+  modules.git.enable = true;
   modules.tmux.enable = true;
   modules.nvim.enable = true;
   modules.claude-code.enable = true;

modules/git.nix

@@ -9,17 +9,8 @@ let
   user = config.user.name;
 in
 {
-  options.modules.git.enable = lib.mkOption {
-    type = lib.types.bool;
-    default = true;
-    description = ''
-      Whether to configure git for the primary user, carrying the operator's
-      commit identity.
-
-      On by default, so every host can commit without restating it. A host that
-      should not carry a personal commit identity sets this to false.
-    '';
-  };
+  options.modules.git.enable =
+    lib.mkEnableOption "git for the primary user, carrying the operator's commit identity";
 
   config = lib.mkIf cfg.enable {
     home-manager.users.${user}.programs.git = {