v0.2.4 — Docs polish

Docs-only release. No functional code change since v0.2.3.

Changed

• Moved promo/ → docs/promo/ so the repository root reads as a clean engineering project.
• README: added a "why three names?" note clarifying repo (EFC-Plugin) / marketplace+package (everything-fact-checked) / plugin (fact-check).
• README: framed efc verify's found verdict as "necessary, not sufficient" — a term-overlap heuristic, not a truth verdict.

All 81 tests pass; all version locations aligned at 0.2.4.