docs: retire PROJECT_GUIDELINES.md — library-visibility rule now in AGENTS.md + dev docs

PROJECT_GUIDELINES.md had drifted out of reality (podman-only rule
while the stack runs docker; verifier scripts wired to Makefile targets
nobody runs). Trashed it, its two verifier scripts, and the Makefile
targets; the rules that still matched reality (docker runtime, schema
in place until release, server binary only via Docker, services layer,
integration-test pool rules) are absorbed into the new AGENTS.md.

The library-visibility family-lens rule now has a real home:
- AGENTS.md: the standing rule agents auto-discover (new media
  endpoints MUST thread the visible-library set; 403, never silently
  pass; history of the 2026-10-13 leak).
- docs/developer/library-visibility.md: the full explanation (helpers,
  SQL pattern, per-item content gates, series per-library), linked
  from the developer docs index.
Stale doc links updated (docs index, contributing, development).

docs/developer/development.md

@@ -73,6 +73,12 @@ Browse individual endpoint documentation with interactive API Explorer:
 
 ## 🛠 Implementation Guides
 
+### Library Visibility
+
+- **[Library Visibility](library-visibility.md)** - Per-user library
+  visibility: the family-lens rule. New media endpoints MUST thread the
+  visible-library set (helpers, SQL pattern, series per-library).
+
 ### Calibre Integration
 
 - **[Calibre OPF Implementation](../development/calibre-opf-implementation.md)** - Technical implementation details