No CI validation for data/last-verified.json

Since #94, `data/last-verified.json` tracks a last-verified date per entry, but nothing checks it. A contributor could commit malformed JSON, a URL that doesn't match any entry in README.md (a typo, or one left behind after an entry was removed), a non-ISO date, or an out-of-alphabetical-order key, and CI would stay green. Add a small validation script (mirroring the style of `scripts/check-list-format.mjs`) that checks: - the file parses as JSON - every key is a URL that appears in one of README.md's entry bullets - every value matches `YYYY-MM-DD` and isn't in the future - keys are sorted alphabetically Wire it into `.github/workflows/lint.yml` alongside the existing checks, and add a test file the way `check-counts.test.mjs` does. **Scope:** medium, roughly 45-60 minutes.