Add cube/box dimension parsing to agent planner

[Copilot]

Requesting a cube (e.g., "build a cube of 30x30x30 mm") fell through to the default template, producing a 30x20x8 rectangular block with a cutout instead of the requested geometry.

Changes

• Dimension parsing — Added _parse_cube_dimensions() to extract dimensions from natural language formats: 30x30x30 mm, 30mm cube, cube of 25
• Box builder — Added _build_box() method to generate sketch + extrude operations with correct dimensions
• Planner routing — Updated execute() and generate_code() to detect cube/box keywords and route appropriately

Example

Before:

# User: "build a cube of 30x30x30 mm"
# Output: .rect(30, 20) with depth=8 and a circular cutout

After:

# User: "build a cube of 30x30x30 mm"  
# Output:
Part(name="Generated Cube").box(30.0, 30.0, 30.0, name="Cube")

Original prompt

---

This section details on the original issue you should resolve

<issue_title>cad model fails to build</issue_title>
<issue_description></issue_description>

<agent_instructions>can you explain the issue and ways to improve</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes cad model fails to build #18

---

📍 Connect Copilot coding agent with Jira, Azure Boards or Linear to delegate work to Copilot in one click without leaving your project management tool.