Document KeepingYouAwake as an App Store-free alternative to Amphetamine

[tcarn]

Summary

Document KeepingYouAwake as an alternative to Amphetamine for users who want to avoid the Mac App Store and signing in with a personal Apple ID on a work Mac.

Proposed documentation

Update README.md §1.2 (Keep the Mac awake) to explain that Amphetamine is the recommended App Store option, but it is not the only option:

### App Store-free alternative: KeepingYouAwake

If you want to avoid the Mac App Store or signing in with a personal Apple ID, install the open-source KeepingYouAwake utility through Homebrew:

```bash
brew install --cask keepingyouawake
open -a KeepingYouAwake

KeepingYouAwake appears in the menu bar. Click its icon and choose a predefined duration, or choose Indefinitely to keep the Mac awake until you turn it off. Use a timed session when possible, and configure its battery protection/start-at-login options if available in the installed version.

KeepingYouAwake covers the basic requirement for this setup: preventing macOS from entering system sleep while the host remains available. It does not provide all Amphetamine features, including app/file-based sessions, Triggers, Drive Alive, closed-lid/clamshell sleep prevention, or Amphetamine's separate display-sleep controls. It also cannot prevent sleep with a MacBook lid closed.

For the server-Mac configuration described here, verify that the display may still sleep and that the screen saver/lock policy behaves as required by the organization's MDM policy. Keep the Mac connected to power for long-running sessions.

## Acceptance criteria

- The prerequisites/setup walkthrough documents KeepingYouAwake as an App Store-free alternative.
- The Homebrew install and launch commands are included.
- Basic menu-bar usage and indefinite/timed sessions are explained.
- The important Amphetamine feature differences and closed-lid limitation are explicit.
- The existing Amphetamine recommendation remains available for users who can use the App Store.