Wire goreleaser, cut v0.1.0-rc1, and verify a6 update end-to-end

[shreemaan-abhishek]

Port a7's .goreleaser.yml to a6 (substitute project name and module path). Add .github/workflows/release.yml that invokes goreleaser on tag push, producing multi-arch binaries (linux / darwin / windows × amd64 / arm64) with sha256 checksums, publishing to GitHub Releases. Cut the first pre-GA tag (v0.1.0-rc1) and verify a6 update works against it end-to-end.

Note: a7 is in the same gap (config exists, no workflow, no tags). A parallel follow-up has been opened against a7.

Part of #33