pkg: clean Homebrew git state

[MikeMcQuaid]

• remove local Git hooks and .gitconfig during .pkg installs
• warn in brew doctor when the Homebrew/brew repo contains them
• avoid local Git customisations breaking installs or updates

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[Copilot]

Pull request overview

This PR hardens Homebrew .pkg installs/updates against local Git customizations by removing repository-local hooks/config and adding a brew doctor warning when they’re detected.

Changes:

• Remove .git/hooks and repository-local .gitconfig during .pkg postinstall repository reset.
• Add a new brew doctor check to warn when Homebrew/brew contains Git hooks or a repo-local .gitconfig.
• Add RSpec coverage for the new diagnostic check (including ignoring *.sample hooks).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
package/scripts/postinstall	Removes repo-local Git hooks and .gitconfig during .pkg installation/reset steps.
Library/Homebrew/diagnostic.rb	Adds check_homebrew_repository_git_hooks to surface problematic Git customizations in brew doctor.
Library/Homebrew/test/diagnostic_checks_spec.rb	Adds unit tests validating the new diagnostic output and .sample hook exclusion.

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