feat: add enclosure creator skill pipeline (Phase 1)

Three chained Claude Code skills:
- enclosure-params: Q&A parameter collection → enclosure-spec.json
- enclosure-generate: JSON → parametric OpenSCAD code
- enclosure-validate: CLI validation, error fixing, STL/PNG export

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