Improve 3D modeling workflow discipline

Add 3D modeling profile with model_3d and render_3d tools ...

New profile:
- modeler_3d: 3D model design for 3D printing (OpenSCAD → STL)

New tools:
- model_3d: compile .scad → binary STL via OpenSCAD CLI
- render_3d: render PNG previews from STL (up to 3 views, 400×300)

Both tools return openscad_not_found error if OpenSCAD is not installed.

Docs:
- README.md: add modeler_3d to profile table
- docs/profiles.md: add modeler_3d to active profiles
- docs/tools.md: add model_3d and render_3d to builtin tools
- manuals/model_3d.md, manuals/render_3d.md: full usage manuals
- modeler_3d system_prompt.txt: OpenSCAD-first workflow with dedicated tools

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>