docs(gates): widen relative-link checker to all docs, repair the rot it finds

TestDocsRelativeLinks walked only docs/developer; widening it to all of
docs/ immediately surfaced the same class of rot the gate was built
for, outside its old reach:

- docs/index.md was the old monolith's landing page: linked the deleted
  api-reference.md, 10 directory-style area links, and escaped the docs
  tree to README/AGENTS/.env/compose. Rewritten as a portal whose every
  link resolves, with one representative endpoint doc per API area and
  pointers to the new writing-ui-docs.md standard.
- directory-style links in user stubs (devices/, ../operations/)
- case-mismatched links (Development.md) in contributing.md
- monolith-era pointer (API_REFERENCE.md, TROUBLESHOOTING.md) and
  escaped AGENTS.md links in contributing/development.md
- docs/development/calibre-opf-implementation.md double-up escapes, a
  link to the removed repo-root plan file (absorbed here), and a
  source-code hyperlink the docs site can't serve (now inline code)
- operations/operations.md linked three never-written guide pages as
  aspirational links - they stay plain text until pages exist

HANDS-OFF (project owner, 2026-11-02): docs/user/filters/** and
docs/user/searching-and-filtering.md are exempt as link sources so no
future checker failure can force edits to the filter documentation.
Files linking INTO them remain checked.

docs/development/calibre-opf-implementation.md

@@ -623,6 +623,6 @@ This is a great example of how user feedback can improve implementation design!
 
 ## See Also
 
-- [User Guide: Calibre Integration](../../user/calibre-integration.md)
-- [Implementation Plan](../../CALIBRE_OPF_IMPLEMENTATION.md)
-- [Scanner Source Code](../../internal/services/media_scanner.go)
+- [User Guide: Calibre Integration](../user/calibre-integration.md)
+- The original implementation plan file was removed from the repo when this page absorbed its content.
+- The scanner implementation lives in `internal/services/` (not hyperlinked — the docs site serves only docs).