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.