Remove the param block and rename DefaultBranch to $defaultBranch. Add argument parsing for iex-compatible invocations (e.g. `iex -dev main`, `iex main`, or no args) by inspecting $args and selecting the branch accordingly, then log the chosen branch. Simplify branch handling and build the GitHub ZIP download URL using the resolved branch. Small cleanup and reordering of steps to accommodate the new parsing logic.
