Add pattern customization via config file

Allow users to customize decision patterns for their specific domain/tech stack.

### Current Limitation
Patterns are hardcoded in decision_detector.py

### Proposed Solution
Support `.devmemory.yml` config file:
```yaml
patterns:
  custom_deployment:
    keywords: [deploy, release, rollout]
    files: [Dockerfile, k8s/]
    weight: 0.8
    emoji: 🚀