automate GitHub Releases for CLI binaries

[rokuosan]

Summary

• add GoReleaser configuration for cross-platform CLI release artifacts
• add a tag-triggered GitHub Actions workflow to publish GitHub Releases on v* tags
• add local validation tasks and document the release flow in the README

Why

The repository already published container images on release tags, but it did not create GitHub Releases for the CLI binary itself. This change makes tagged releases publish versioned binaries and checksums alongside the existing container image flow.

Validation

• go test ./...
• verified that v* tags are wired to both the existing container workflow and the new GoReleaser workflow
• documented local dry-run commands via task release:check and task release:snapshot