Add zsh + oh-my-zsh, set default shell per distro family

- Add zsh to SYSTEM_PACKAGES
- Detect RHEL-family vs Debian-family from /etc/os-release ID/ID_LIKE
  (falls back to PKG_MGR if os-release is unreadable)
- ensure_zsh_default sets zsh as the invoking user's login shell after
  system install: usermod -s on RHEL-family (chsh under default
  authselect refuses other-user changes), chsh -s elsewhere; SUDO_USER
  is preferred so sudo invocations target the real user
- Add oh-my-zsh to CUSTOM_PACKAGES; the installer runs the official
  unattended script (requires zsh + git, which are already installed by
  this point) and rewrites ZSH_THEME to "gnzh" in ~/.zshrc
- As a final step, run 'zsh -c "source ~/.zshrc"' to validate the rc
  file (the user's interactive shell is unaffected — they need a new
  terminal to pick up the new default shell)

formatted_packages.py

@@ -52,6 +52,7 @@ SYSTEM_PACKAGES: list[str] = [
     "wireshark",
     "yt-dlp",
     "zoom",
+    "zsh",
     "bzip2",
     "bzip2-devel",
     "gdbm-libs",
@@ -111,4 +112,5 @@ CUSTOM_PACKAGES: list[dict] = [
     {"name": "nvm"},
     {"name": "pyenv"},
     {"name": "pip"},
+    {"name": "oh-my-zsh"},
 ]