feat: Dredd → PromptForge SOUL refinement pipeline

[DarlingtonDeveloper]

Implements the NATS-based refinement proposal system as specified in briefing d851c9d5.

What's included

1. NATS Refinement Consumer (prompt_forge/core/refinement/consumer.py)

• Subscribes to pattern.refinement.proposed events from NATS
• Creates refinement branches with format refinement/{section}/{timestamp}
• Applies proposed changes to target sections
• Handles errors gracefully with proper logging

2. Proposals Endpoint (prompt_forge/api/proposals.py)

• GET /api/v1/prompts/{slug}/proposals endpoint
• Lists pending refinement branches (active branches with refinement/ prefix)
• Returns branch name, creation time, target section, and source patterns
• Integrated into main API router

3. NATS Integration

• Wired refinement consumer into main app startup/shutdown lifecycle
• Uses existing NATS connection pattern (NATS_URL env var)
• Gracefully degrades if NATS is unavailable

Testing

• All existing 290 tests continue to pass
• New functionality tested for correct import and instantiation
• Section change application logic verified

Implementation Details

• Follows existing codebase patterns and conventions
• Error handling matches existing subscribers
• Branch creation uses existing VCS methods
• Optional metadata storage for source patterns (backwards compatible)