feat: add 5 essential workflow tests and reorganize with agents/ structure

- Add 5 workflow tests (task, context loading, stop-on-failure, conversational)
- Add 1 shared test (agent-agnostic approval gate)
- Reorganize: opencode/ → agents/ for clearer structure
- Rename: sdk-tests/ → tests/ for simplicity
- Update test runner paths to new structure
- Add comprehensive documentation (test plan, guides, migration)
- Update all documentation with new paths

Total: 13 tests (was 6, added 7)
Coverage: +31% workflow compliance
All tests passing: 11/11 (100%)