docs(author): add strategy-selection reference with empirical contract ladder (#1810)

Companion deep reference for the SKILL.md strategy gate (#1809):

- New `references/strategy-selection.md` with contract-based ladder model,
  empirical fixes/adapter-year data (837 adapters / 30-day window), Pattern A
  judgment rules from `api_candidates` verdicts, and reference cases
  (booking #1680, Twitter GraphQL, xhs signed URL, weread-official).
- Cross-link from SKILL.md inline strategy gate to the deep reference, plus
  one-line empirical hook ("PAGE_FETCH/INTERCEPT 7-8x PUBLIC_API fix rate").
- coverage-matrix.md: Strategy row renamed to 6-enum (PUBLIC_API / COOKIE_API
  / UI_SELECTOR / DOM_STATE / PAGE_FETCH / INTERCEPT) with fixes/adapter-year
  on each entry.
- site-recon.md: Pattern A note that hit alone is not a `PAGE_FETCH` signal —
  must check `api_candidates` verdicts (booking #1680 reference).