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/index.md

@@ -63,7 +63,7 @@ Complete guide to Bookhoard documentation. Find what you need quickly.
 **[Contributing Portal](contributing/contributing.md)** - Development workflow
 
 - [Development Guide](contributing/development.md) - Architecture, setup, testing
-- [../PROJECT_GUIDELINES.md](../PROJECT_GUIDELINES.md) - Development rules and standards
+- [../../AGENTS.md](../../AGENTS.md) - Development rules and standards
 
 ---
 
@@ -143,7 +143,7 @@ When adding new features:
 3. **Backend changes** → Update [Development Guide](contributing/development.md)
 4. **Deployment changes** → Update [Operations Portal](operations/operations.md)
 
-Keep [../PROJECT_GUIDELINES.md](../PROJECT_GUIDELINES.md) in mind for documentation standards.
+Keep [../../AGENTS.md](../../AGENTS.md) in mind for documentation standards.
 
 ---