feat(grpo): add pluggable reward functions / verifier registry

[abrichr]

reward.py has only binary_task_success with no extensibility. RL training use cases need custom per-task verifiers and reward composition.

Current reward is hardcoded in rollout_collector.py:140.

Proposed design:

• Implement a reward function protocol matching TRL's reward_funcs pattern (list of callables)
• Support a TaskVerifierRegistry for registering task-specific verification functions