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_SEARCH_IMPLEMENTATION.md

@@ -1065,7 +1065,7 @@ If there's a section about documentation features, mention the new search functi
 
 ## Git Commit Strategy
 
-Follow PROJECT_GUIDELINES.md - make multiple logical commits:
+Follow AGENTS.md conventions - make multiple logical commits:
 
 1. **Backend implementation** (search.go, http_handler.go, router changes)
 2. **Unit tests** (search_test.go)