packaging: remove obsolete homebrew-formula.json manifest

[cybertec-adrian]

scripts/homebrew-formula.json was a leftover hand-maintained tap manifest from an earlier formula approach that nothing consumes — the Homebrew artefact is generated and pushed to the tap by goreleaser on release. Delete it and update scripts/README.md to point at the homebrew_casks stanza in .goreleaser.yaml instead.

Summary

Remove the obsolete scripts/homebrew-formula.json (a hand-maintained tap manifest nothing consumes; the Homebrew artefact is generated by goreleaser) and update scripts/README.md.

Type

• Bug fix
• New feature
• Refactor (no behaviour change)
• Documentation
• Packaging / release

Tests

• make check passes locally (vet + race tests)
• New tests added for the changed behaviour, or there's a clear reason none exist
• Integration tests pass (make test-integration) where touched

Compatibility

• No on-disk manifest schema changes (or: schema bumped + 24-month back-read preserved)
• No CLI / API contract changes (or: documented + bumped)
• No new external dependencies (or: justified in the description)

Checklist

• Maintainer-attribution authoring (Author: Hans-Jürgen Schönig <hs@cybertec.at>)
• No AI-attribution lines anywhere
• CHANGELOG.md updated under the unreleased section
• Comments explain WHY (not WHAT) where the code isn't self-evident