dev/sg: fix macos firewall exceptions

Created by: bobheadxi

• Make binary detection more robust: shell expansion, split to get first command, etc.
  • Make Caddy binary file naming simpler to enable this
• More helpful output when adding things to the firewall
• Code clarity improvements

Test plan

go run ./dev/sg start

see if things have been added/are already added, see no popups 👌