Refactor blindspot-check to write-a-skill standard

- Trim SKILL.md 149 -> 100 lines via progressive disclosure: move the
  big-commitment checks and trade validity bar into
  references/decision_playbooks.md
- Fix description spec violations: third person, literal "Use when"
  trigger phrase, 982 chars parsed (was 1057, over the 1024 cap), all
  quoted triggers preserved
- Correct bare antidotes.md reference to references/antidotes.md
- Fix doubled wording in bias_catalogue.md Hindsight Bias entry
- Update README file tree for the new reference file

Verified: write-a-skill checklist 6/6 PASS; 4-lens adversarial review
plus final verifier confirm no procedural content lost.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>