203 - Implementation Design

Produce .specflow/features/<fid>-<feature-slug>/implementation.md: a standalone implementation plan that tells an engineer what files should change, why they change, and how the feature should fit the existing codebase.

The core rule is reuse-first: prefer extending existing modules over creating new files. A new file needs explicit justification.

Output path: .specflow/features/<fid>-<feature-slug>/implementation.md