User-domain agents are not rediscovered from the user LaunchAgents directory after reboot. Let built-in agents inherit the GUI default while keeping explicit user-domain configuration available.
User-domain agents need the Background session type to bootstrap into user/501, and activation should fail rather than silently unloading services when launchctl bootstrap fails.
On Darwin, launchd may attempt to start agents before the Nix store is
mounted and available. This leads to failures when the agent's executable
or arguments reside in the Nix store.
This change wraps the agent's command in a shell script that uses
/bin/wait4path to ensure /nix/store is ready before executing the
original program. It also ensures that ProgramArguments are correctly
escaped and concatenated.
Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
