feat(orchestrator): add test workflow engine placeholder

Initial scaffold for the test workflow engine service directory.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: frostebite

src/model/orchestrator/services/test/README.md

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+# Test Workflow Engine
+
+Service for YAML-based test suite definitions, taxonomy filtering, and structured test results.
+
+See GitHub Issue for full specification.