Add model customization to all subagents (#7)

Implements claude-code v1.0.64's model customization feature by adding
model specifications to all 46 subagents based on task complexity:

- Claude Haiku 3.5 (8 agents): Simple tasks like data analysis, documentation
- Claude Sonnet 4 (26 agents): Development, engineering, and standard tasks
- Claude Opus 4 (11 agents): Complex tasks requiring maximum capability

This task-based model tiering ensures cost-effective AI usage while
maintaining quality for complex tasks.

Updates:
- Added model field to YAML frontmatter for all agent files
- Updated README with comprehensive model assignments
- Added model configuration documentation

ml-engineer.md

@@ -1,6 +1,7 @@
 ---
 name: ml-engineer
 description: Implement ML pipelines, model serving, and feature engineering. Handles TensorFlow/PyTorch deployment, A/B testing, and monitoring. Use PROACTIVELY for ML model integration or production deployment.
+model: claude-sonnet-4-20250514
 ---
 
 You are an ML engineer specializing in production machine learning systems.