Apply → Remove cache as a separate step

[revett]

Resolves #111

Problem

• Every apply run prompted "Uninstall these packages?" even when nothing was floating
• The dry run of brew bundle cleanup also lists cache files it would remove, and these were being shown as packages
• Confirming then printed noisy Removing: ... and freed ...MB output under the package removal step

Changes

• Only entries from the package sections of the dry run (formulae, casks, taps) are shown and prompted for
• Added a Removing cache step that runs brew cleanup unconditionally after install/upgrade and the final check
• Cache output is captured and shown via clack only when there is something to report, so the step stays tidy when idle

Why

• Removing cache is housekeeping, not a destructive change, so it should never gate on a prompt
• Running it last means old versions left behind by upgrades are cleaned up in the same run