Add externalConfig option to support external openclaw.json files

[TorosFanny]

Summary

This PR adds a new externalConfig option to the nix-openclaw home-manager module, allowing users to specify an external openclaw.json file instead of generating config solely from Nix options.

Motivation

The nix-openclaw module generates openclaw.json from Nix code using schema-typed options. However, some channels (like Matrix) are not yet supported in the generated options. Users who need these unsupported channels must either:

1. Manually maintain openclaw.json outside of home-manager (risk of being overwritten)
2. Fork the module to add missing options

This PR provides a third option: use an external openclaw.json file directly while still benefiting from home-manager's declarative configuration.

Changes

• Add externalConfig option in options-instance.nix:

  • Type: nullOr str
  • Accepts JSON content as a string (use builtins.readFile to read from file)

• Update config.nix to:

  • Check if externalConfig is set
  • If set, use the external config directly
  • Add validation to prevent using both config and externalConfig simultaneously

Usage Example

programs.openclaw = {
  enable = true;
  instances.default = {
    # Use external config file
    externalConfig = builtins.toJSON (builtins.fromJSON (builtins.readFile ./openclaw.json));
  };
};

Backwards Compatibility

This change is fully backwards compatible. Existing configurations using config = { ... } will continue to work without any modifications.