feat(images): wire image pipeline into article flow (commit 1b) (#14)

Completes the image + A/B experiment workstream by integrating
image generation into the post publishing pipeline.

New classes:
- class-image-pipeline.php: Orchestrates A/B generation (article-driven
  Image A → featured; source-driven Image B → post meta). Each image
  independently fallible; article publishes even if both fail.
- class-image-prompt-builder.php: Synthesizes visual prompts from article
  title+content and Reddit source data. Appends CEO-locked 90s infomercial
  style suffix. Keeps prompts concise for FLUX.1 quality.
- class-image-media-sideloader.php: Downloads image bytes, creates temp
  file, sideloads via media_handle_sideload(), sets alt text and metadata,
  cleans up temp files.

Changes to existing files:
- Publisher.attach_generated_images(): Runs image pipeline post-creation,
  sets _thumbnail_id (Image A) and _prautoblogger_image_b_id (Image B).
  Errors logged but don't block post publication.
- Settings: prautoblogger_image_enabled (toggle, default off).

Tests:
- ImagePipelineTest.php: Orchestration, cost tracking, graceful failures.
- ImagePromptBuilderTest.php: Prompt generation, length limits.

Docs:
- ARCHITECTURE.md: New step 6b in data flow, new files in file tree.
- CHANGELOG.md: Commit 1a/1b breakdown.

Co-authored-by: peptiderepo <peptiderepo@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/Core/ImagePromptBuilderTest.php

@@ -0,0 +1,122 @@
+<?php
+/**
+ * Tests for PRAutoBlogger_Image_Prompt_Builder.
+ *
+ * Validates prompt generation from article content and source data,
+ * including style suffix appending and HTML stripping.
+ *
+ * @package PRAutoBlogger\Tests\Core
+ */
+
+namespace PRAutoBlogger\Tests\Core;
+
+use PRAutoBlogger\Tests\BaseTestCase;
+use Brain\Monkey\Functions;
+
+class ImagePromptBuilderTest extends BaseTestCase {
+
+	protected function setUp(): void {
+		parent::setUp();
+
+		// Mock WordPress functions.
+		Functions\when( 'wp_strip_all_tags' )->alias( function ( $str ) {
+			return wp_strip_all_tags( $str );
+		} );
+		Functions\when( 'sanitize_text_field' )->returnArg();
+		Functions\when( 'get_option' )->alias( function ( $key, $default = false ) {
+			static $options = [];
+			return $options[ $key ] ?? $default;
+		} );
+	}
+
+	/**
+	 * Test build_article_prompt() generates a prompt from article title and content.
+	 */
+	public function test_build_article_prompt_from_content(): void {
+		$builder = new \PRAutoBlogger_Image_Prompt_Builder();
+
+		$article_data = [
+			'post_title'   => 'How to Train Your Dragon',
+			'post_content' => '<p>Dragons are fascinating creatures. Learn the secrets.</p>',
+		];
+
+		$prompt = $builder->build_article_prompt( $article_data );
+
+		$this->assertStringContainsString( 'How to Train Your Dragon', $prompt );
+		$this->assertStringContainsString( 'Dragons are fascinating', $prompt );
+		// Style suffix should be appended.
+		$this->assertNotEmpty( $prompt );
+		$this->assertGreaterThan( 50, strlen( $prompt ) );
+	}
+
+	/**
+	 * Test build_article_prompt() handles missing content gracefully.
+	 */
+	public function test_build_article_prompt_with_missing_content(): void {
+		$builder = new \PRAutoBlogger_Image_Prompt_Builder();
+
+		$article_data = [
+			'post_title' => 'Minimal Article',
+		];
+
+		$prompt = $builder->build_article_prompt( $article_data );
+
+		$this->assertStringContainsString( 'Minimal Article', $prompt );
+		$this->assertNotEmpty( $prompt );
+	}
+
+	/**
+	 * Test build_source_prompt() generates a prompt from Reddit data.
+	 */
+	public function test_build_source_prompt_from_reddit_data(): void {
+		$builder = new \PRAutoBlogger_Image_Prompt_Builder();
+
+		$source_data = [
+			'title'    => 'Help: My Robot Won\'t Stop Dancing',
+			'comments' => [
+				'Have you tried unplugging it?',
+				'Maybe it\'s just happy.',
+			],
+		];
+
+		$prompt = $builder->build_source_prompt( $source_data );
+
+		$this->assertStringContainsString( 'Robot', $prompt );
+		$this->assertStringContainsString( 'Dancing', $prompt );
+		$this->assertNotEmpty( $prompt );
+	}
+
+	/**
+	 * Test build_source_prompt() handles missing comments.
+	 */
+	public function test_build_source_prompt_without_comments(): void {
+		$builder = new \PRAutoBlogger_Image_Prompt_Builder();
+
+		$source_data = [
+			'title' => 'Standalone Title',
+		];
+
+		$prompt = $builder->build_source_prompt( $source_data );
+
+		$this->assertStringContainsString( 'Standalone Title', $prompt );
+		$this->assertNotEmpty( $prompt );
+	}
+
+	/**
+	 * Test that prompts don't exceed reasonable length.
+	 */
+	public function test_prompts_are_reasonably_sized(): void {
+		$builder = new \PRAutoBlogger_Image_Prompt_Builder();
+
+		$long_content = '<p>' . str_repeat( 'a', 5000 ) . '</p>';
+		$article_data = [
+			'post_title'   => 'Test',
+			'post_content' => $long_content,
+		];
+
+		$prompt = $builder->build_article_prompt( $article_data );
+
+		// Prompt should be under 500 chars (concept + style suffix).
+		$this->assertLessThan( 500, strlen( $prompt ) );
+	}
+}