Create a Homebrew installer script

Reference: https://docs.brew.sh/How-to-Create-and-Maintain-a-Tap

Start by reading the linked Homebrew guide and checking how gitlabctl currently distributes its executable. The issue names no files or tests; done means a working Homebrew installation path for this Go CLI, with the packaging behavior verified.

由索引模型根據 Issue 內容生成。