add opencrow Matrix bot on ful

Uses upstream NixOS module (systemd-nspawn container with sandboxing).

- User: @fable:4d2.org on matrix.4d2.org
- Provider: GitHub Copilot (OAuth, one-time interactive login)
- pkgs.pi added via extraPackages (available on PATH inside container)
- Secrets via agenix: matrix token as environmentFile,
  soul bind-mounted into the container
- Sessions + pi-agent state in /var/lib/opencrow, backed up via restic

To complete setup:
1. Create secrets/opencrow-matrix-token.age (OPENCROW_MATRIX_ACCESS_TOKEN=...)
2. Create secrets/opencrow-soul.age (SOUL.md content)
3. One-time Copilot login inside the container:
   sudo nixos-container root-login opencrow
   PI_CODING_AGENT_DIR=/var/lib/opencrow/pi-agent pi
   # Run /login, select GitHub Copilot, complete OAuth flow

systems/ful/opencrow.nix

@@ -16,6 +16,8 @@
   services.opencrow = {
     enable = true;
 
+    extraPackages = [ pkgs.pi ];
+
     environmentFiles = [
       config.age.secrets.opencrow-matrix-token.path
     ];