chore(release): package and distribute the kubeuser CLI binary

[MuhanedYahya]

Why

Part of the self-service kubeconfig bootstrap initiative (tracking issue TBD, linked once created). Users need a way to install the CLI before kubeuser init means anything.

Scope

• Decide build/release tooling (e.g. goreleaser) producing cross-platform binaries (linux/darwin, amd64/arm64).
• Checksummed release artifacts attached to GitHub Releases.
• Install instructions (curl script and/or package manager — exact channel TBD, doesn't block earlier sub-issues).

Acceptance criteria

• Tagged release produces binaries for the target platforms with checksums
• Documented install path

Depends on

#106 (init CLI), #108 (sync CLI) — needs both commands to exist for a meaningful first release

[MuhanedYahya]

Part of #111