refactor(orchestrator): Introduce WorkflowState dataclass (Priority 7) (#129)

refactor(factory): Implement Provider Registry Pattern (Priority 6) (#128)

refactor(config): Remove dead config and Anthropic references (Priority 4)

fix(P0): Auto-detect OpenAI provider from BYOK api_key prefix

test: Update mock messages to include contents attribute

feat(SPEC-16): Unified Chat Client Architecture (#115)