feat: refresh shared skills catalog

skills/cad-3d-print/SKILL.md

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+---
+name: cad-3d-print
+description: >
+  Design, generate, and verify parametric 3D models for 3D printing using code-first CAD (OpenSCAD, build123d).
+scope: global
+---
+
+# CAD 3D Print
+
+Design, generate, and empirically verify 3D-printable parts using programmatic CAD.
+Follow the [shared workflow](../../protocols/workflow.md).
+
+## Workflow
+
+1. **Dimensional Budgeting & Tolerances**
+   - Establish bounding box, mounting points, and mating interfaces from user specifications.
+   - Apply standard FDM 3D printing tolerances:
+     - **Horizontal hole clearance:** Add `+0.2 mm` to `+0.3 mm` for screw pass-through (e.g. M3 hole diameter = `3.2 mm`; tap hole = `2.9 mm`).
+     - **Fit clearances:** Snug/snap fit `0.15 mm`; sliding fit `0.3 mm`; loose/hinged fit `0.4–0.5 mm`.
+     - **Wall thickness:** Multiples of nozzle width (minimum `1.2 mm` for 0.4 mm nozzles).
+     - **Bed contact:** Add 45° chamfers to bottom edges touching the build plate to prevent "elephant's foot" binding.
+     - **Overhangs:** Keep unassisted overhangs $\le 45^\circ$ to minimize support material.
+
+2. **Parametric Script Generation**
+   - Author a standalone, self-documenting OpenSCAD (`.scad`) script:
+     - Place all customizable dimensions at the top as labeled parametric variables.
+     - Set curvature resolution: `$fn = 60;` or `$fs = 0.5; $fa = 5;` for smooth circular features.
+     - Use Constructive Solid Geometry (CSG): `union()`, `difference()`, `intersection()`, `hull()`.
+     - Prevent non-manifold coincident faces (Z-fighting): always extend cutting bodies slightly beyond the target surface (e.g., `translate([0, 0, -0.1]) cylinder(h = height + 0.2, ...)`).
+
+3. **Empirical Verification**
+   - **Compile STL mesh:**
+     ```bash
+     openscad -o model.stl model.scad
+     ```
+     Inspect compiler output to ensure `Simple: yes` (a manifold, watertight solid).
+   - **Render visual preview:**
+     ```bash
+     openscad --render --imgsize=800,600 -o preview.png model.scad
+     ```
+   - Verify the rendered image to check geometry, clearances, and aesthetic proportions.
+
+4. **Handoff Artifacts**
+   - Keep the editable `.scad` source in the workspace.
+   - Provide the compiled `.stl` or `.3mf` for slicing (PrusaSlicer, Bambu Studio, OrcaSlicer).
+   - Embed or link the rendered `.png` preview.