fix(homebrew): rewrite bump-homebrew.sh to handle multi-arch formulas (#60)

[scott]

Summary

• brew bump-formula-pr can't update URLs nested inside on_macos / on_linux blocks (Homebrew core's stance is "not supported" — Homebrew/brew#8967). Since Formula/gander.rb ships 4 separate binaries (macOS+Linux × arm64/amd64), the original scripts/bump-homebrew.sh errored with Could not find url stanza!.
• Replace the brew bump-formula-pr call with an inline Ruby rewrite that:
  • reads current version from the formula URL
  • fetches every release asset's SHA256 via gh release view --json assets
  • rewrites v<old> → v<new> in every URL and updates the matching sha256
  • tightens the test do block from --help to --version (idempotent; closes test(homebrew): tighten Formula/gander.rb test to --version once next release ships #59)
  • branches, commits, pushes, and gh pr creates against the tap

End-to-end verification (closes #60)

• PR gandermd/homebrew-gander#2 opened against the tap with the expected diff (URLs + SHA256s + tightened test).
• Audit CI on the bump PR is green.
• Post-merge brew install gandermd/gander/gander installs v0.12.0; brew test gandermd/gander/gander passes the new --version assertion.
• brew audit --strict --new --tap=gandermd/gander gander exits 0.

Closes

• Closes chore(homebrew): exercise scripts/bump-homebrew.sh end-to-end on next release #60 — first non-dry-run of scripts/bump-homebrew.sh validated end-to-end.
• Closes test(homebrew): tighten Formula/gander.rb test to --version once next release ships #59 — formula test now asserts gander --version (which gander gained in v0.12.0).

Notes

• The brew preflight check is gone — it's no longer required. The script now requires gh (authenticated, repo scope) and ruby (ships with macOS).
• One stale tap gotcha: git push then immediately brew install on the same checkout can leave the tap cache showing merge-conflict markers from the in-flight push. brew untap && brew tap (or rm -rf $(brew --repository)/Library/Taps/gandermd/homebrew-gander) clears it.