feat: add backend-testing skill with comprehensive Encore MCP patterns

- Create backend-testing_skill with focus on API testing using Encore MCP
- Add three comprehensive reference docs:
  * encore_mcp_testing_patterns.md - 10 testing patterns with MCP tools
  * api_testing_examples.md - ScreenGraph service-specific test examples
  * integration_test_patterns.md - Multi-service integration testing
- Update backend_vibe.json skill description for clarity
- Update qa_vibe.json skill description with comprehensive details
- Skill covers API contract testing, trace-driven debugging, and service integration validation

Author: nirukk52