301 - Spec-Driven System Implementation

Implement a substantial change with this strict phase order:

1. Prepare or refresh feature context
2. Write failing tests
3. Implement the minimum production code to pass
4. Clean up tests
5. Clean up production code
6. Run full validation

Do not mix test and production cleanup in the same pass.

Output: Update source and test files in the project. If .specflow/features/<fid>-<feature-slug>/overview.md exists, change status from todo to implementing when work starts and to done only after full validation passes. If 201/202/203/204 artifacts had to be synthesized for this run, keep them in working memory unless the user explicitly asks to persist them.