feat: implement SDK-based evaluation framework with real agent testing

- Replace planned framework with working SDK implementation using @opencode-ai/sdk
- Add 6 YAML test cases across developer/business/edge-case categories (100% passing)
- Implement 4 evaluators for openagent.md rule compliance (approval-gate, context-loading, delegation, tool-usage)
- Add real-time event streaming and session recording
- Configure opencode/grok-code-fast as default model (zero API costs)
- Add comprehensive documentation (SDK_EVAL_README.md, test-design-guide.md, TEST_SCENARIOS.md)
- Support v2 schema with behavior-based expectations instead of message counts
- Enable model-agnostic test design that works across Claude, GPT-4, and Grok
- Add subagent invocation syntax documentation to openagent.md