Import 13 skills from the old opencode setup into turnstone

skills/ carries the SKILL.md files; scripts/import_skills.py runs
inside the VM via turnstone's own storage layer (SKILL.md parser +
risk scanner) so rows match what the console admin API would write.
Server must be restarted after import (stale WAL snapshot).

skills/3d-modeling-openscad/SKILL.md

@@ -0,0 +1,68 @@
+---
+name: 3d-modeling-openscad
+description: Generate parametric 3D models using OpenSCAD script
+license: WTFPL
+compatibility: opencode
+metadata:
+  audience: makers, engineers, designers
+  workflow: 3d-modeling
+---
+
+## What I do
+
+- Write `.scad` files using OpenSCAD syntax (primitives, transformations, CSG).
+- Create parametric models using modules, functions, and variables.
+- Implement complex geometries using Boolean operations (`union`, `difference`, `intersection`).
+- Apply modifiers like `$fn` for smoothness or `center` for alignment.
+- Create scripts for procedural 3D design.
+- **Execute a Visual Verification Loop**:
+    - Render `.scad` to `.png` using the OpenSCAD CLI.
+    - Use the `read` tool to inspect the rendered image.
+    - Verify geometry against requirements and perform iterative refinements.
+
+## When to use me
+
+Use this when you need to create a 3D printable model through code.
+Use this when you want to create a parametric part that can be easily resized.
+Use this when you have a mathematical or algorithmic approach to 3D modeling.
+
+## Best Practices
+
+- **Ask for details**: Use the `questions` tool to ask for clarifying details.
+  - For example: What aspects of the model should be parameterized?
+
+## Boolean subtractions
+**Avoid 3D boolean subtractions**. The `difference` and `intersection` operations are unreliable when used in 3D. Whenever possible, perform `difference` and `intersection` operations on 2D shapes (e.g., `circle`, `square`) before using `linear_extrude` to create 3D volumes. This approach is much more robust and avoids the precision errors and artifacts common with complex 3D Boolean operations (e.g., when creating a tube or a hollow cylinder).
+
+Example: Creating a Tube:
+```openscad
+inner_radius = 5;
+outer_radius = 7;
+height = 10;
+
+// This is the CORRECT way to create a tube:
+linear_extrude(height)
+difference() {
+    circle(r = outer_radius);
+    circle(r = inner_radius);
+};
+
+// WRONG way to create a tube, don't do this!!! `difference` on 3d objects should be avoided!
+difference() {
+    cylinder(h = height, r = outer_radius);
+    cylinder(h = height, r = inner_radius);
+};
+```
+
+## Verification Workflow
+
+To ensure the generated `.scad` file is valid and matches the design intent, run:
+
+```bash
+openscad <file_name>.scad -o <file_name>.render.png
+```
+
+**Purpose:**
+1. **Syntax Check**: If the command fails, the script contains errors that must be fixed.
+2. **Visual Validation**: Use the `read` tool on the resulting `.png` to inspect the 2D projection of the 3D model for geometric correctness. **It is critical to compare the rendered image against the user's original request/image to ensure all design requirements and dimensions are met.**
+```