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/contributing/contributing.md

@@ -4,7 +4,7 @@ Welcome to the Bookhoard contributing documentation. This section contains guide
 
 ## 🚀 Getting Started
 
-- **[Development Guide](Development.md)** - Development workflow and architecture
+- **[Development Guide](development.md)** - Development workflow and architecture
   - Architecture overview
   - Directory structure
   - Local development setup
@@ -14,7 +14,7 @@ Welcome to the Bookhoard contributing documentation. This section contains guide
 
 ## 📋 Project Guidelines
 
-- **[AGENTS.md](../../AGENTS.md)** - Agent/development rules
+- **[AGENTS.md](../contributing/contributing.md)** - Development rules and standards live in the Contributing portal
   - Critical prohibitions
   - Mandatory requirements
   - Error recovery protocol
@@ -22,7 +22,7 @@ Welcome to the Bookhoard contributing documentation. This section contains guide
 
 ## 🤝 How to Contribute
 
-We welcome contributions! Please see our [Development Guide](Development.md) for information on:
+We welcome contributions! Please see our [Development Guide](development.md) for information on:
 
 - Setting up your development environment
 - Understanding the codebase