Evaluate `tdd` skill in real use

[bracyw]

Follow-up from the Matt Pocock skills adoption PR (#646).

Trial the tdd skill on a real feature or bug fix and confirm the red-green-refactor loop genuinely helps rather than adding friction.

Acceptance

• Use /tdd end-to-end on at least one real change.
• Note where the red-green-refactor enforcement helped vs. felt heavy.
• Tune .claude/skills/tdd/SKILL.md (or leave a comment recommending no change) based on findings.

Tooling-evaluation follow-up; not blocking.