Implement redact and route_local actions:

Implement redact and route_local actions:

- Integrate redact action: redacts secrets before PII detection, unredacts in responses
- Implement route_local action: routes requests with secrets to local provider
- Update stream transformer to handle both PII and secrets contexts
- Add comprehensive tests for secrets routing logic
- Update config.example.yaml with new entity types and action documentation
- Update README.md with complete secrets detection features

New secret entity types (opt-in):

- API_KEY_OPENAI, API_KEY_AWS, API_KEY_GITHUB
- JWT_TOKEN, BEARER_TOKEN

Response headers:
- X-PasteGuard-Secrets-Redacted: true (when action=redact)