chore(homebrew): exercise scripts/bump-homebrew.sh end-to-end on next release

[scott]

Summary

scripts/bump-homebrew.sh <version> was added in PR #58 to automate the
Homebrew tap bump after each scripts/release.sh cut. So far only the
--dry-run path has been validated locally; the non-dry path (real
brew bump-formula-pr, real PR opened against gandermd/homebrew-gander)
has not been exercised.

Why this matters

The dry-run verifies argument parsing and the GitHub release existence check,
but not:

• brew bump-formula-pr actually fetching fresh SHA256s from the release
  assets and rewriting Formula/gander.rb
• gh opening a PR against gandermd/homebrew-gander with the diff
• .github/workflows/audit.yml (Homebrew audit) passing on the resulting PR
• The resulting formula actually installing on a fresh brew install

If any of those fail, we want to find out before users are waiting on a
release, not after.

Acceptance criteria

• Cut the next release via scripts/release.sh <version> as usual
• Immediately run scripts/bump-homebrew.sh <version> (no --dry-run)
• Confirm a PR was opened against gandermd/homebrew-gander
• Confirm the audit workflow runs and is green
• On a separate machine / under brew install --build-from-source, confirm the bumped formula installs and passes brew test
• Merge the bump PR
• Run brew upgrade gander on a Homebrew-installed copy and confirm it picks up the new version

Notes

• Pre-flight: brew installed locally and gh auth status reports write
  access to gandermd/homebrew-gander. The script checks both and exits
  clearly otherwise.
• The gander --version test tightening (test(homebrew): tighten Formula/gander.rb test to --version once next release ships #59) should land in the same
  bump PR — they're sequenced on the same release.

[scott]

Done end-to-end against the v0.12.0 release:

• scripts/bump-homebrew.sh 0.12.0 (rewritten in PR fix(homebrew): rewrite bump-homebrew.sh to handle multi-arch formulas (#60) #61, since brew bump-formula-pr can't traverse on_macos/on_linux blocks)
• PR gander 0.12.0 homebrew-gander#2 opened with correct URL + SHA256 + test-tightening diff
• Audit CI green
• Merged; post-merge brew install gandermd/gander/gander + brew test both pass