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/blog/src/entities/errors/article.ts

@@ -3,9 +3,3 @@ export class ArticleNotFoundError extends Error {
     super(message, options);
   }
 }
-
-export class InputParseError extends Error {
-  constructor(message: string, options?: ErrorOptions) {
-    super(message, options);
-  }
-}

packages/blog/src/entities/errors/common.ts

@@ -0,0 +1,5 @@
+export class InputParseError extends Error {
+  constructor(message: string, options?: ErrorOptions) {
+    super(message, options);
+  }
+}

packages/blog/src/entities/errors/errors.test.ts

@@ -1,5 +1,6 @@
 import { describe, expect, it } from "vitest";
-import { ArticleNotFoundError, InputParseError } from "./errors";
+import { ArticleNotFoundError } from "./article";
+import { InputParseError } from "./common";
 
 describe("ArticleNotFoundError", () => {
   it("uses the default message when none is given", () => {