Add Prompt Duel Optimizer (PDO) - Label-Free Prompt Optimization

[heyjustinai]

What does this PR do?

Introduces Prompt Duel Optimizer (PDO) - a label-free prompt optimization strategy based on our research paper (arXiv:2510.13907). PDO uses dueling bandits and Thompson sampling to optimize prompts without requiring labeled validation data.

Key Features:

• Label-free optimization using LLM judge for pairwise comparisons
• Double Thompson Sampling for efficient prompt selection
• Top-performer guided mutation for prompt evolution
• Outperforms baselines on BIG-bench Hard and MS MARCO

What's Included:

• Core PDO implementation in src/prompt_ops/core/pdo/
• Example config: configs/pdo-example.yaml
• Use case: use-cases/web-of-lies-pdo/ (logical reasoning benchmark)
• Updated README and documentation

Test Plan

cd use-cases/web-of-lies-pdo
prompt-ops migrate --config config.yaml

Full test: Run with default 30 rounds for complete benchmark results.