feat: add GAN-style generator-evaluator harness (#1029)

Implements Anthropic's March 2026 harness design pattern — a multi-agent
architecture that separates generation from evaluation, creating an
adversarial feedback loop that produces production-quality applications.

Components:
- 3 agent definitions (planner, generator, evaluator)
- 1 skill with full documentation (skills/gan-style-harness/)
- 2 commands (gan-build for full apps, gan-design for frontend)
- 1 shell orchestrator (scripts/gan-harness.sh)
- Examples and configuration reference

Based on: https://www.anthropic.com/engineering/harness-design-long-running-apps

Co-authored-by: Hao Chen <haochen806@gmail.com>

examples/gan-harness/README.md

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+# GAN-Style Harness Examples
+
+Examples showing how to use the Generator-Evaluator harness for different project types.
+
+## Quick Start
+
+```bash
+# Full-stack web app (uses all three agents)
+./scripts/gan-harness.sh "Build a project management app with Kanban boards and team collaboration"
+
+# Frontend design (skip planner, focus on design iterations)
+GAN_SKIP_PLANNER=true ./scripts/gan-harness.sh "Create a stunning landing page for a crypto portfolio tracker"
+
+# API-only (no browser testing needed)
+GAN_EVAL_MODE=code-only ./scripts/gan-harness.sh "Build a REST API for a recipe sharing platform with search and ratings"
+
+# Tight budget (fewer iterations, lower threshold)
+GAN_MAX_ITERATIONS=5 GAN_PASS_THRESHOLD=6.5 ./scripts/gan-harness.sh "Build a todo app with categories and due dates"
+```
+
+## Example: Using the Command
+
+```bash
+# In Claude Code interactive mode:
+/project:gan-build "Build a music streaming dashboard with playlists, visualizer, and social features"
+
+# With options:
+/project:gan-build "Build a recipe sharing platform" --max-iterations 10 --pass-threshold 7.5 --eval-mode screenshot
+```
+
+## Example: Manual Three-Agent Run
+
+For maximum control, run each agent separately:
+
+```bash
+# Step 1: Plan (produces spec.md)
+claude -p --model opus "$(cat agents/gan-planner.md)
+
+Your brief: 'Build a retro game maker with sprite editor and level designer'
+
+Write the full spec to gan-harness/spec.md and eval rubric to gan-harness/eval-rubric.md."
+
+# Step 2: Generate (iteration 1)
+claude -p --model opus "$(cat agents/gan-generator.md)
+
+Iteration 1. Read gan-harness/spec.md. Build the initial application.
+Start dev server on port 3000. Commit as iteration-001."
+
+# Step 3: Evaluate (iteration 1)
+claude -p --model opus "$(cat agents/gan-evaluator.md)
+
+Iteration 1. Read gan-harness/eval-rubric.md.
+Test http://localhost:3000. Write feedback to gan-harness/feedback/feedback-001.md.
+Be ruthlessly strict."
+
+# Step 4: Generate (iteration 2 — reads feedback)
+claude -p --model opus "$(cat agents/gan-generator.md)
+
+Iteration 2. Read gan-harness/feedback/feedback-001.md FIRST.
+Address every issue. Then read gan-harness/spec.md for remaining features.
+Commit as iteration-002."
+
+# Repeat steps 3-4 until satisfied
+```
+
+## Example: Custom Evaluation Criteria
+
+For non-visual projects (APIs, CLIs, libraries), customize the rubric:
+
+```bash
+mkdir -p gan-harness
+cat > gan-harness/eval-rubric.md << 'EOF'
+# API Evaluation Rubric
+
+### Correctness (weight: 0.4)
+- Do all endpoints return expected data?
+- Are edge cases handled (empty inputs, large payloads)?
+- Do error responses have proper status codes?
+
+### Performance (weight: 0.2)
+- Response times under 100ms for simple queries?
+- Database queries optimized (no N+1)?
+- Pagination implemented for list endpoints?
+
+### Security (weight: 0.2)
+- Input validation on all endpoints?
+- SQL injection prevention?
+- Rate limiting implemented?
+- Authentication properly enforced?
+
+### Documentation (weight: 0.2)
+- OpenAPI spec generated?
+- All endpoints documented?
+- Example requests/responses provided?
+EOF
+
+GAN_SKIP_PLANNER=true GAN_EVAL_MODE=code-only ./scripts/gan-harness.sh "Build a REST API for task management"
+```
+
+## Project Types and Recommended Settings
+
+| Project Type | Eval Mode | Iterations | Threshold | Est. Cost |
+|-------------|-----------|------------|-----------|-----------|
+| Full-stack web app | playwright | 10-15 | 7.0 | $100-200 |
+| Landing page | screenshot | 5-8 | 7.5 | $30-60 |
+| REST API | code-only | 5-8 | 7.0 | $30-60 |
+| CLI tool | code-only | 3-5 | 6.5 | $15-30 |
+| Data dashboard | playwright | 8-12 | 7.0 | $60-120 |
+| Game | playwright | 10-15 | 7.0 | $100-200 |
+
+## Understanding the Output
+
+After each run, check:
+
+1. **`gan-harness/build-report.md`** — Final summary with score progression
+2. **`gan-harness/feedback/`** — All evaluation feedback (useful for understanding quality evolution)
+3. **`gan-harness/spec.md`** — The full spec (useful if you want to continue manually)
+4. **Score progression** — Should show steady improvement. Plateaus indicate the model has hit its ceiling.
+
+## Tips
+
+1. **Start with a clear brief** — "Build X with Y and Z" beats "make something cool"
+2. **Don't go below 5 iterations** — The first 2-3 iterations are usually below threshold
+3. **Use `playwright` mode for UI projects** — Screenshot-only misses interaction bugs
+4. **Review feedback files** — Even if the final score passes, the feedback contains valuable insights
+5. **Iterate on the spec** — If results are disappointing, improve `spec.md` and run again with `--skip-planner`