Default to headless install; add --gui flag; add lazygit and pulumi

- Inverts the GUI opt-out model: GUI packages and Flatpak are now skipped
  by default (headless-friendly), and --gui opts in to installing them.
  Removes --no-gui entirely.
- Adds lazygit and pulumi to the default system package list.

https://claude.ai/code/session_01CsTAu2SNhE5ZAQm3RnVwp3

formatted_packages.py

@@ -36,6 +36,7 @@ SYSTEM_PACKAGES: list[str] = [
     "git",
     "github-desktop",
     "google-chrome-stable",
+    "lazygit",
     "lua",
     "minisign",
     "minikube",
@@ -43,6 +44,7 @@ SYSTEM_PACKAGES: list[str] = [
     "obsidian",
     "pipx",
     "poetry",
+    "pulumi",
     "podman",
     "qemu",
     "restic",