docrot v1.8.0 — local-repo scanner + docrot-check action

Added

• scan_local.py + tests (6) — local-repo scanner for CI and GitHub
  Action use. Same classification rules as scan.py (in-repo / missing /
  external / unresolvable) but operates on a checked-out tree, so a CI
  step can run the same engine on its own repository without a GitHub
  token. Headline figure: externalThirdPartyBroken + missingInRepo.
  test_scan_local.py covers a 9-image fixture mixing dead external,
  missing in-repo, and healthy refs; the live-CI path is gated by
  SKIP_LIVE_CHECKS=1.
• CI: dogfoods auroraxo/docrot-check (warn-only) so the published
  engine scans the engine's own docs. Pinned to the action's v1 tag
  now that the tagged release exists.
• README "Screened, not rot" section — explicit list of the three
  in-repo README.md images that look like dead rot (a favicon that
  points to a non-200, a local asset path that's only resolved by the
  docs site, and a sponsor placeholder) but are actually intentional
  and reproducible; documents the live host-aggregation result for
  deepwiki/repology/starchart so a future reader doesn't file a
  duplicate issue.
• README "Known rot clusters" section — eight cluster hosts
  (travis-ci.org legacy, broken.stock, dead-knight, etc.) with the
  number of distinct target URLs and their live HTTP status, plus the
  upstream issues filed against the affected projects.

Changed

• CI test matrix now runs test_scan_local alongside the existing
  test_docrot and test_version_drift suites (70 + 19 + 6 = 95 tests on
  Python 3.10/3.12).
• docrot-check pin in the docs CI moved from a hard-coded commit
  SHA to the action's published v1 tag.