Systemd user services don't inherit shell environment. Add PATH with
essential tools (git, coreutils, gnugrep, gnused, findutils) so opencode
can run basic commands.
- Add opencode/web.nix module for web service configuration
- Bind to Tailscale IP (100.122.32.106) on port 4096
- Pass JUSPAY_API_KEY from agenix secret to service
- Update flake inputs: nixpkgs/unstable, nix-darwin/master, home-manager/master