feat(git): personal email and commitizen aliases

Set user.email = iam@emmathe.dev on the personal hosts (mkDefault, so the
work module's address still wins on the work box). Add git aliases for
commitizen -- `git cz <sub>` (e.g. `git cz c`) and `git cc` for the commit
prompt; commitizen is already installed on every host (home.packages) and
defaults to the Conventional Commits ruleset.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyrathorpe/home/git.nix

@@ -16,6 +16,9 @@
     package = pkgs.gitFull;
     settings = {
       user.name = fullName;
+      # Personal identity. mkDefault so the work module overrides it on the work
+      # host (and to merge cleanly with that plain definition there).
+      user.email = lib.mkDefault "iam@emmathe.dev";
       push.autoSetupRemote = true;
       init.defaultBranch = "main";
 
@@ -48,6 +51,10 @@
         last = "log -1 HEAD";
         unstage = "reset HEAD --";
         lg = "log --graph --abbrev-commit --decorate --format=format:'%C(bold blue)%h%C(reset) %C(bold green)(%ar)%C(reset) %C(white)%s%C(reset) %C(dim white)- %an%C(reset)%C(auto)%d%C(reset)' --all";
+        # commitizen (Conventional Commits, its default ruleset): `git cz c` ->
+        # `cz commit`, `git cz bump`, etc. `git cc` is a shortcut for the prompt.
+        cz = "!cz";
+        cc = "!cz commit";
       };
 
       # SSH commit signing on personal hosts too (the work module sets the same