[httpsphl]

Closes one of the Linux/macOS gaps (spec 14.1): Keep the computer awake while bots work now works off Windows.

• platform/unix/power.rs: while any bot is busy (and the setting is on), a helper process holds the request.
  • macOS: caffeinate -i -w <daemon pid>.
  • Linux: systemd-inhibit --what=idle --mode=block around a sh that waits for the daemon to go away.
• Like the Windows power request, it only stops sleep after inactivity; the lid and Sleep still work. The helper ends with the daemon, so a crash does not keep the computer awake.
• A helper that exits within 200 ms, as systemd-inhibit does with no logind, returns an error, which the caller logs.
• The old keep-awake test is Windows-only now. The new Unix test skips where no helper can run: it skipped in the Docker container, which has no logind. The macOS CI job runs it for real with caffeinate.
• The README and the code signing policy describe 0.8.0, where this is Windows-only, so they change with the next release.

Manual test (not done yet)

• macOS:
  1. With a bot working, pmset -g assertions lists caffeinate.
  2. Once the bot is idle, it is gone.
• Linux: the same check with systemd-inhibit --list showing Botloft.