feat: add devtu-self-evolve skill, refactor oversized skills for progressive disclosure (#120)

- New devtu-self-evolve skill: orchestrates full ToolUniverse improvement cycle
  (discover → create → test → fix → optimize → ship)
- Refactor devtu-optimize-skills (1466→133 lines): move patterns, testing standards,
  case studies, and checklists to references/
- Refactor devtu-auto-discover-apis (1356→124 lines): move tool templates and
  validation guide to references/
- Refactor devtu-create-tool (949→221 lines): move testing guide to references/
- Remove auxiliary files (README.md, QUICK_START.md, etc.) per skill-creator guidelines

Author: gasvn