Add a guided Engineering Brief workflow for AI-assisted design

[10-X-eng]

User outcome

Give engineers a guided way to turn an incomplete natural-language request into a clear, reviewable engineering brief before the CAD agent starts acting.

Proposed experience

Add an optional Build Engineering Brief action to the VibeCAD bar beside the normal send workflow.

1. Open a dedicated dialog that the user can keep visible and read comfortably.
2. Seed it with the user's request plus safe context already known by VibeCAD: active document, selection, workspace, units, and cached document context.
3. Let a non-mutating brief assistant ask only high-value missing questions, one at a time.
4. Show the evolving brief throughout the conversation in a readable, editable preview.
5. Allow Use best judgment for unanswered details and show every resulting assumption explicitly.
6. Let the user approve the brief with Start in VibeCAD; hand the original request, approved brief, assumptions, and document references atomically to the existing agent.
7. Keep ordinary chat/send behavior unchanged for users who do not use the feature.

Brief content

• Objective and desired deliverable
• Existing geometry/document context
• Dimensions and units
• Materials
• Interfaces and constraints
• Loads and operating conditions
• Manufacturing process and tolerances
• Required analyses/drawings
• Acceptance criteria
• Explicit assumptions and open questions

Product rules

• The brief assistant cannot mutate CAD or silently invent consequential engineering values.
• It must not ask for information VibeCAD already knows.
• It must distinguish hard requirements from preferences.
• It must not question forever: the user can proceed at any point with explicit assumptions.
• The canonical brief should be structured data; the user sees and edits a natural-language rendering.
• Persist the brief with the document/conversation so it can be reopened and revised.
• Normal VibeCAD chat and existing provider/tool contracts remain compatible.

Acceptance criteria

• A visible VibeCAD-bar action opens a readable Engineering Brief window.
• A user can enter a rough goal, answer guided questions, and see the brief update.
• The window exposes editable brief content and explicit assumptions/open questions.
• The workflow uses active document context without mutating the document.
• The user can stop questioning and proceed with best judgment.
• Starting work creates one clear handoff to the normal VibeCAD agent.
• Closing/canceling the window does not send or mutate anything.
• Existing normal chat behavior remains unchanged.
• The feature is covered by red/green tests, including persistence and handoff behavior.