refactor(features): split entities into models/ + errors/ subdirs

All 5 features (auth, blog, marketing-pages, navigation; media has no
entities yet) now follow Lazar's pattern:
- entities/<x>.ts → entities/models/<x>.ts
- entities/errors.ts → entities/errors/<domain>.ts + errors/common.ts

Updates all import paths across factories, contracts, tests, use cases,
controllers, repositories, integrations, and src/index.ts exports.

navigation divergence: had no errors.ts; errors/header.ts +
errors/common.ts added as new forward-looking stubs.

Refactor log: docs/superpowers/refactor-logs/2026-05-05-lazar-pattern-conformance.md
Spec: §5, §9.3

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/auth/src/entities/errors/auth.ts

@@ -0,0 +1,17 @@
+export class AuthenticationError extends Error {
+  constructor(message: string, options?: ErrorOptions) {
+    super(message, options);
+  }
+}
+
+export class UnauthenticatedError extends Error {
+  constructor(message: string, options?: ErrorOptions) {
+    super(message, options);
+  }
+}
+
+export class UnauthorizedError extends Error {
+  constructor(message: string, options?: ErrorOptions) {
+    super(message, options);
+  }
+}