Add write-brd skill, build inputs, and reference skills
- .claude/skills/write-brd: the built skill (thin SKILL.md, 7 reference files, 9 output templates, Neil AP product KB shipped as reference)
- brd-skill-prompt.md (authoritative spec), write-brd-research-distilled.md (research handoff), research-report.md (full survey), NEIL-AP-PRODUCT.md
- .claude/skills/write-prd and interrogate: sibling reference skills
- README describing the repo; validation phase pending
Co-Authored-By: Claude Fable 5 noreply@anthropic.com