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.
Add configuration options for the opencode web service including host,
port, mDNS, logging, and CORS settings. Implement systemd user service
to run the web server with configurable parameters.