design-explore/image-maker: use free Ming Image 0.1 Design via /api/v1/images

- Ming (inclusionai/ming-image-0.1-design) is free and built for graphic-design
  output with legible in-image text; now the preferred mockup model.
- Ming is a true image-generation model: the chat/completions endpoint rejects
  it (HTTP 404). Documented the /api/v1/images call + base64 decode recipe.
- Kept paid chat-image models as a documented fallback.
- Fixed the API key location: OPENROUTER_API_KEY is in /home/sam/.secrets,
  NOT ~/.config/environment.d/10-secrets.conf as the old text claimed.

skills/design-explore/SKILL.md

@@ -7,6 +7,8 @@ description: Generate visual UI design mockups as images for exploration and ide
 
 Generate visual UI mockups. Asks the user what to design, pulls design tokens or style references, and calls `image-maker` to produce high-fidelity mockup images.
 
+> **Image model:** use `inclusionai/ming-image-0.1-design` — it is **free**, and it is specifically built for graphic-design output with legible text rendering inside the image, which is exactly what UI mockups need. It is a true image-generation model and must be called on OpenRouter's `/api/v1/images` endpoint (NOT `/chat/completions`). See the `image-maker` subagent for the exact call.
+
 ---
 
 ## 🎨 Visual System & Image Prompting Rules
@@ -32,11 +34,13 @@ curl -sL <DESIGN_MD_URL> -o /tmp/design-tokens.md
 
 ### Step 3: Call `image-maker`
 
+Instruct it to use the free Ming model via the images endpoint.
+
 ```bash
 Agent(
   subagent_type: "image-maker",
   description: "Generate design mockup images",
-  prompt: "High-fidelity UI mockup for {what}. Aesthetic: {vibe/tokens}. Clean browser frame, non-generic layout, bold typography, rich contrast, professional SaaS/web app finish.",
+  prompt: "Use model inclusionai/ming-image-0.1-design via POST https://openrouter.ai/api/v1/images (free). Generate a high-fidelity UI mockup for {what}. Aesthetic: {vibe/tokens}. Clean browser frame, non-generic layout, bold typography, rich contrast, professional SaaS/web app finish. Save as /tmp/design-explore-<N>.png and report the path.",
   max_turns: 3
 )
 ```