feat(agent): clarify vague briefs before generating and harden prompts

Let the planner return a clarify/chat intent that asks 2-4 targeted
questions (imageCount 0) when essentials are missing, instead of forging
ahead. Tighten prompt guidance to preserve user wording and stated
reference-image roles, avoid inventing copy/specs, keep durable brand
constraints, and treat vague taste feedback as clarification. Also
fix the generation heartbeat goroutine to stop cleanly via defer, add
research-query fallbacks, and refresh image-only capability copy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/infrastructure/piagent/design_agent.go

@@ -402,9 +402,9 @@ func withReferenceImageInstruction(prompt string, imageCount int) string {
 		return prompt
 	}
 	if prefersEnglish(prompt) {
-		return "Input reference image(s) are attached. Use them as the visual source of truth for product shape, fabric, color, structure, and identity; when extracting, continuing, or editing a product, do not invent a different item.\n" + prompt
+		return "Input reference image(s) are attached. Use each image only for the role stated by the user, such as product/source, logo, style, color, composition, or edit target. If editing or preserving a source product, keep its identity; do not invent unstated roles from the image content.\n" + prompt
 	}
-	return "已随请求附带参考图；生成时必须以参考图中的商品外形、材质、颜色、结构细节和身份为准，拆出、延续或编辑商品时不要另造不同商品。\n" + prompt
+	return "已随请求附带参考图；只按用户原话说明的用途使用每张图，例如商品/源图、Logo、风格、色彩、构图或编辑对象。需要编辑或延续源商品时保持其身份，不要仅凭图片内容臆测未说明的角色。\n" + prompt
 }
 
 func imagePromptTooLarge(prompt string) bool {