Add more builtin agents (e.g. mobile_developer, ml_engineer)

[jee599]

Currently we have 9 builtin agents. Adding more specialized agents would improve auto-dispatch quality.

How to add:

1. Create a YAML file in agents/builtin/
2. Follow the existing format (name, role, description, identity, critical_rules, deliverables, success_metrics)
3. Run npm test to verify

Good candidates:

• mobile_developer (React Native / Flutter)
• ml_engineer (model training, MLOps)
• database_engineer (schema design, migrations)
• technical_pm (sprint planning, backlog)