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/contributing/contributing.md

@@ -14,7 +14,7 @@ Welcome to the Bookhoard contributing documentation. This section contains guide
 
 ## 📋 Project Guidelines
 
-- **[PROJECT_GUIDELINES.md](../PROJECT_GUIDELINES.md)** - Development rules and standards
+- **[AGENTS.md](../../AGENTS.md)** - Agent/development rules
   - Critical prohibitions
   - Mandatory requirements
   - Error recovery protocol

docs/contributing/development.md

@@ -445,7 +445,7 @@ DEBUG=true
 
 ## 📚 Additional Resources
 
-- [Project Guidelines](../../PROJECT_GUIDELINES.md) - Development rules and standards
+- [AGENTS.md](../../AGENTS.md) - Agent/development rules
 - [API Reference](../API_REFERENCE.md) - Complete API documentation
 - [Troubleshooting](../TROUBLESHOOTING.md) - Deployment issues
 - [Go Documentation](https://go.dev/doc/)