Migrate the Homebrew tap to grate-devs/homebrew-cask

[erikbra]

Context

Follow-up to #796. The grate cask currently lives in erikbra/homebrew-cask and is installed as erikbra/cask/grate. The canonical tap should be owned by the grate-devs organization.

Scope

• Create or transfer the tap history to grate-devs/homebrew-cask.
• Publish the current grate cask as grate-devs/cask/grate.
• Provide a Homebrew-native migration from erikbra/cask/grate, using tap_migrations.json in the old tap if that is the selected approach.
• Update the cask to use canonical grate-devs/grate release URLs and the current documentation homepage.
• Configure cross-repository release automation using a narrowly scoped GitHub App or fine-grained token; the grate repository's GITHUB_TOKEN cannot write to another repository.
• Validate the cask on Apple Silicon and Intel.
• Add appropriate audit/syntax checks to the tap or release workflow.

Acceptance criteria

• brew install --cask grate-devs/cask/grate succeeds.
• Both Apple Silicon and Intel downloads have correct checksums.
• Existing users of erikbra/cask/grate have a tested migration path.
• A stable grate release can update the organization-owned cask automatically.
• Prereleases do not update the stable cask.
• Cross-repository credentials are organization-manageable and least-privileged.
• Documentation changes are coordinated with the distribution documentation sweep.

References

• Improve the release story #796
• Homebrew tap migrations: https://docs.brew.sh/Migrating-A-Formula-To-A-Tap
• GitHub repository transfers: https://docs.github.com/repositories/creating-and-managing-repositories/transferring-a-repository