feat: add GoReleaser and release workflow for distribution

[timimsms]

Summary

Adds automated release pipeline so cu can be distributed via Homebrew and direct download.

• GoReleaser v2 config (.goreleaser.yml) for cross-platform binary builds
• GitHub Actions release workflow (.github/workflows/release.yml) triggered on version tags

What this enables

After merge, creating a release is:

git tag v0.1.0
git push origin v0.1.0

This automatically:

• Builds binaries for Linux (amd64, arm64), macOS (amd64, arm64), Windows (amd64)
• Creates a GitHub Release with archives and checksums
• Generates and pushes a Homebrew Cask to timimsms/homebrew-cu

Verified locally

• goreleaser check passes with no warnings
• goreleaser build --snapshot --clean builds all 5 platform binaries
• goreleaser release --snapshot --clean --skip=publish generates archives, checksums, and Homebrew cask

Manual setup required before first release

1. Create the tap repository: timimsms/homebrew-cu on GitHub (with a Casks/ directory)
2. Create a PAT with repo scope and add as HOMEBREW_TAP_GITHUB_TOKEN secret on the cu repository
3. Optionally tag the first release: git tag v0.1.0 && git push origin v0.1.0

Test plan

• goreleaser check - config valid, no deprecation warnings
• goreleaser build --snapshot --clean - all 5 binaries build
• goreleaser release --snapshot --clean --skip=publish - full pipeline works
• CI passes
• First real release after merge (manual step)

🤖 Generated with Claude Code