Choose Brew behavior during chezmoi updates

[ETeissonniere]

Summary

• expose one-off Brew behavior overrides through DOTFILES_BREW_MODE=upgrade|install|skip
• keep ordinary chezmoi update, dotsync, and bootstrap behavior unchanged
• let install mode add missing packages without a general upgrade, or skip Brew entirely
• render the selected mode into the on-change hook so a one-time skip does not permanently mark package changes as reconciled

Usage

• DOTFILES_BREW_MODE=install chezmoi update
• DOTFILES_BREW_MODE=skip chezmoi update

Validation

• mocked Brew execution verified upgrade, install, and skip behavior
• install mode invokes brew bundle --no-upgrade --file=-
• rendered Bash syntax passes for upgrade, install, skip, and invalid modes
• each mode renders a distinct content hash
• isolated chezmoi init/apply dry-runs pass for all three supported modes
• zsh syntax check, ShellCheck, and git diff --check pass

Homebrew documents that --no-upgrade suppresses the general upgrade step, though installing a missing package may still require dependency upgrades.