feat(cli): initialize Homebrew taps

[jmgilman]

Summary

• add release-cli init homebrew-tap for deterministic cask-only tap scaffolds
• pin generated tap validation to the running CLI source commit
• reject nonempty outputs and invalid development build stamps before mutation
• document tap repository, GitHub App, branch protection, and producer setup

Verification

• go test ./internal/cli -run 'TestInitHomebrewTap' -count=1
• mise exec -- moon run root:check --summary minimal
• end-to-end stamped CLI invocation generated the expected four-file scaffold
• generated casks.yml matches meigma/homebrew-tap after normalizing the existing missing final newline

Trust boundaries

• initializer performs local filesystem writes only; no Git or GitHub mutation
• generated workflow has top-level permissions: {} and job-scoped contents: read
• workflow revision comes from the full source commit stamped into the released CLI