fix: configure macOS shutdown hook

[lucaspin]

Adds a shutdown hook to terminate the instance when the agent shuts down.

The shutdown hook is executed in a non-login shell. Since LaunchD has no default environment generator mechanism like systemD, we'd need to use the launchctl setenv or launchctl config commands for configuring it.

Since the only environment setup needed in that script is to include /usr/local/bin to the PATH, we do it in the script itself, for simplicity.