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

@@ -0,0 +1,5 @@
+export class ArticleNotFoundError extends Error {
+  constructor(message = "Article not found", 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

@@ -0,0 +1,24 @@
+import { describe, expect, it } from "vitest";
+import { ArticleNotFoundError } from "./article";
+import { InputParseError } from "./common";
+
+describe("ArticleNotFoundError", () => {
+  it("uses the default message when none is given", () => {
+    const err = new ArticleNotFoundError();
+    expect(err).toBeInstanceOf(Error);
+    expect(err.message).toBe("Article not found");
+  });
+
+  it("uses a custom message when provided", () => {
+    const err = new ArticleNotFoundError("could not find article abc");
+    expect(err.message).toBe("could not find article abc");
+  });
+});
+
+describe("InputParseError", () => {
+  it("is an instance of Error with the given message", () => {
+    const err = new InputParseError("invalid input");
+    expect(err).toBeInstanceOf(Error);
+    expect(err.message).toBe("invalid input");
+  });
+});