atom-nix: `cfg` as json api

[nrdxp]

Implement the Nix-side configuration input mechanism, establishing a typed JSON interface for receiving client-side configuration into the atom-nix evaluation context.

Design

The cfg scope will:

• Receive pre-validated JSON configuration via a well-defined schema
• Expose configuration values to the module scope
• Support both static values and dynamically generated content by accepting any JSON that validates against the schema (e.g. build matrices)
• Provide a clean interface for external tools to inject configuration before evaluation

Implementation

1. Introduce cfg scope at the top-level
2. Define JSON schema that specifies:
   • Valid configuration structure
   • Type constraints
   • Required vs optional fields
3. Implement JSON ingestion and exposure to module system

Technical Benefits

Input Boundary

• Creates clear interface for external configuration injection
• Moves type checking responsibility to client side, eliminating complex validation logic from Nix evaluation context
• Enables clean HTTP API integration for remote evaluation via pre-validated JSON, leveraging pervasive JSON schema and web API library support across ecosystems

Architecture

• Separates configuration concerns from evaluation logic
• Provides typed interface without the debugging complexity inherent to Nix-based validation
• Establishes foundation for plugin-based schema extensions
• Supports optimization of evaluation scheduling through clean configuration boundaries

Deliverables

• Initial cfg scope implementation with JSON ingestion
• Base schema definition for atomic dependency management and fundamental configuration
• Consider an interface to extend Base