Add cache-homebrew-prefix action

[MikeMcQuaid]

Caches the Homebrew prefix, installs packages via brew install and keys the cache from brew list --versions.

[Copilot]

Pull request overview

Introduces a new composite GitHub Action to cache the Homebrew prefix, install requested packages via brew install, and save the prefix cache keyed from the installed package versions.

Changes:

• Added a cache-homebrew-prefix composite action implementing restore → install → hash → save workflow.
• Added documentation for usage, inputs, outputs, and behavioral notes.

Reviewed changes

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

File	Description
cache-homebrew-prefix/action.yml	Implements the composite action steps for restoring/saving a Homebrew prefix cache and installing packages.
cache-homebrew-prefix/README.md	Documents how to use the new composite action, including inputs/outputs and caveats.

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