Add robotics CAD robustness guidelines and prompt add-on

[BenCaunt]

Motivation

• Improve the robustness of LLM-generated robotics CAD by documenting common failure modes (missing fillets, unsupported cantilevered shafts, weak bosses) and proposing concrete guardrails.
• Provide a repeatable prompt and workflow so generated parts prioritize interfaces, serviceability, and validated support topology rather than only layout or aesthetics.
• Seed a practical roadmap toward deterministic feature templates, a geometry linter, and an intent-to-geometry solver pipeline that preserve LLM productivity while reducing mechanical risk.

Description

• Add docs/robotics-design-guidelines.md containing design philosophy, frequent failure modes with remedies, a staged prompt contract, tooling recommendations, and a validation checklist.
• Add skills/synthcad-cad-authoring/references/robotics-robustness-prompt.md as a reusable prompt add-on that enforces guardrails and requires reasoning output (interface map, load-path, template plan, risks) before emitting code.
• Extend skills/synthcad-cad-authoring/SKILL.md with a Robotics Robustness Extension section that points to the new reference files and mandates explicit rationale for rotating supports and load-path decisions.
• The guidelines include actionable next steps (e.g., adding synthcad/cad/lint.py, deterministic constructors, CI integration) to be implemented in follow-up work.

Testing

• Ran the test suite with uv run pytest -q.
• Test collection failed in this environment due to a missing system dependency (libGL.so.1) required by build123d/OCP, preventing further automated verification.
• No repository-level tests were changed; the new docs and prompt files do not affect runtime behavior and should pass CI once the environment provides the required system libraries.

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Codex Task

[github-actions]

SynthCAD CAD review

Compared 50cc8c6e6505 to 5a220def1175 for flat-disk-robot.

• Interactive diff viewer
• Workflow run and generated artifacts