fix: authenticate Homebrew tap Git operations

[robjtede]

The Homebrew updater previously called gh auth setup-git, but Git could still select an existing Actions credential. The tap push then ran as github-actions[bot] instead of the GitHub App and failed with HTTP 403.

This change resets Git's configured credential helpers for each remote tap operation and explicitly uses gh auth git-credential. That helper reads the updater's GH_TOKEN, so clone, branch lookup, fetch, and push all use the intended GitHub App installation token.

The release-tool fixture now rejects Homebrew branch pushes that do not include the explicit helper configuration.

Validation: just check.

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