generate_3d: 3D models with fal.ai (image, text, or image steered by text)

- New tool: image-to-3D (project file or an image attached to the chat,
  optional back/left/right views) or text-to-3D; image + prompt refines the
  image with the prompt first (the 3D models take no text with an image).
  Styles textured / geometry / lowpoly, optional PBR, face count. Runs through
  fal.ai's queue API with progress, saves GLB + preview, books the cost
  (published prices, overridable per model). Request bodies for the Hunyuan3D,
  Tripo and Trellis families; defaults [images] model_3d / model_3d_text.
- Chat card: "Open in the 3D view"; media overview: kind "model" (also
  STL/3MF/OBJ written by agents) with previews.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -176,6 +176,10 @@ output limit, reasoning effort, extra request options and prices per million tok
 - **Images (fal.ai):** the agent drafts cheap previews (FLUX schnell), asks which one to
   refine, and creates the final image with the best model for the purpose – Nano Banana Pro
   refines the chosen draft, Ideogram handles lettering, Recraft design work.
+- **3D models (fal.ai):** `generate_3d` turns an image (also one pasted into the chat, plus
+  optional back/left/right views), a text prompt, or an image steered by a prompt into a GLB
+  model – textured, untextured for 3D printing, or low-poly (Hunyuan3D v3 by default,
+  `[images] model_3d` / `model_3d_text`). Open it in the 3D view of the file manager.
 
 ## Terminal client