RFC: Generator-Evaluator Dual-Agent Architecture (Harness Mode)

Motivation

The current Ralph is a highly effective single-agent loop. After extensive real-world use and battle-testing (especially on Windows/MSYS2), I've developed a significant enhancement: a Generator-Evaluator dual-agent architecture that adds contract negotiation, phase discipline, and 4-dimension quality scoring.

This RFC proposes adding "Harness Mode" as an optional architecture alongside the existing simple mode.

Key Concept

In Harness Mode, two specialized AI agents collaborate per story — Generator (builds, never judges) and Evaluator (judges, never builds):

1. Contract Negotiation — Generator proposes scope → Evaluator reviews → back-and-forth until locked
2. Build — Generator reads locked contract → implements → typecheck/lint/test → commits
3. Evaluate — Evaluator tests in browser (Playwright MCP) → scores on 4 dimensions → writes evaluation.json
4. Retry or Advance — Pass all thresholds → next story. Fail → feedback to Generator → retry

Proposed Changes

New (4 files): generator-prompt.md, evaluator-prompt.md, contract.json.example, AGENTS.md Modified (1 file): ralph.sh — purely additive, run_simple_mode() untouched

Default mode remains simple — fully backward compatible:

./ralph.sh                           # simple mode (unchanged)
./ralph.sh --mode harness --tool claude  # new harness mode

Questions

1. Is the dual-agent architecture direction aligned with your vision?
2. Any concerns about naming or structure before I submit a PR?
3. Would you prefer harness code in ralph.sh or a separate script?

Happy to iterate based on your feedback.