refactor: rename lib/ to core/ for clearer core-module separation

Rename the top-level lib/ directory to core/ so the project root
immediately communicates which code is core platform and which lives
in modules/. PHP namespaces (MintyPHP\*) are unchanged — only the
Composer PSR-4 path mapping moves from lib/ to core/.

Module-internal lib/ directories (modules/*/lib/) are untouched.

Updated across the full stack:
- composer.json PSR-4 mapping
- bootstrap entry points (web/index.php, bin/*, tests/bootstrap.php)
- tooling configs (phpstan.neon, phpunit.xml, php-cs-fixer)
- 26 architecture contract tests
- enforcement-policy, guard-catalog, quality-gates
- all documentation (CLAUDE.md, README, 25 docs/, .agents/skills/)
- bin/qa-extended.sh search paths
- .claude/settings.local.json permission paths

Workflow: .agents/runs/CORE-LIB-RENAME-001/ (Analyst → Planner →
Executor → Code Review (4 findings fixed) → Security Review →
Acceptance Test → Finalizer)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/Repository/User/UserExternalIdentityRepositoryInterface.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace MintyPHP\Repository\User;
+
+/** Contract for linking external identity provider accounts (OIDC) to users. */
+interface UserExternalIdentityRepositoryInterface
+{
+    public function findByProviderTidOid(string $provider, string $tid, string $oid): ?array;
+
+    public function findByProviderIssuerSubject(string $provider, string $issuer, string $subject): ?array;
+
+    public function createLink(array $data): mixed;
+}