feat: Wave 2 eval harness + image pipeline instrumentation (v0.13.0)

Closes #119

- Extract PRAutoBlogger_Image_Scene_Parser from image prompt builder (331→300 lines)
- Add frozen eval dataset (20 peptide topics at eval/dataset.json)
- Add LLM-as-judge scorer (gemini-flash-1.5 via OpenRouter, 3 axes)
- Add eval runner orchestrator with dry-run and Opik push support
- Add WP-CLI command: wp prautoblogger opik:eval [--limit=<n>] [--dry-run]
- Add image pipeline Opik instrumentation (image_prompt_rewrite span)
- Add content generator eval mode (suppresses publish + image gen)
- Version: v0.13.0

includes/core/class-content-generator.php

@@ -29,7 +29,10 @@ class PRAutoBlogger_Content_Generator {
 	 * @param PRAutoBlogger_Article_Idea $idea The scored idea to generate content for.
 	 * @return string Generated HTML content.
 	 */
-	public function generate( PRAutoBlogger_Article_Idea $idea ): string {
+	public function generate( PRAutoBlogger_Article_Idea $idea, bool $eval_mode = false ): string {
+		// In eval mode, suppress publishing and image generation side effects.
+		define( 'PRAUTOBLOGGER_EVAL_MODE', $eval_mode );
+
 		$mode = get_option( 'prautoblogger_writing_pipeline', 'multi_step' );
 
 		$request = new PRAutoBlogger_Content_Request(
@@ -285,4 +288,14 @@ class PRAutoBlogger_Content_Generator {
 
 		return $response['content'];
 	}
+
+	/**
+	 * Generate content in eval mode (no publishing, no image generation).
+	 *
+	 * @param PRAutoBlogger_Article_Idea $idea The scored idea to generate content for.
+	 * @return string Generated HTML content.
+	 */
+	public function generate_eval( PRAutoBlogger_Article_Idea $idea ): string {
+		return $this->generate( $idea, true );
+	}
 }