Adds `examples/launchd` with a plist and the steps to run the runner on macOS, next to the existing systemd example.
The runner runs in the login session of the user that jobs should run as, which gives jobs the login keychain and the GUI. The README also covers the two problems that setup runs into: macOS blocks the runner from hosts on the Mac's own subnet, and Git's keychain helper can stop a job at a dialog.
_Written by Claude (Fable 5.2)._
Reviewed-on: https://gitea.com/gitea/runner/pulls/1408
Reviewed-by: bircni <[email protected]>
Co-authored-by: silverwind <[email protected]>
