Add Homebrew formula and release automation

[nicholasgcoles]

Summary

Make TabGate easy to install via Homebrew and automate the release pipeline.

Tasks

• Add --version flag to the binary
• Create a Makefile with build, install, test, clean targets
• Set up GoReleaser for cross-compilation and GitHub Releases
• Create a Homebrew tap (homebrew-tap repo) with formula
• Add GitHub Actions CI for tests + release pipeline

Goal

brew install nic/tap/tabgate
# or
go install github.com/nic/tabgate@latest