Design and implement custom prompts for project templates

[TomMaSS]

Summary

Each project template (Blank, SaaS MVP, Mobile App, API Design) currently creates a basic section structure. We should design rich starter prompts/content for each template that give users a meaningful head start.

Templates to design

Template	Current State	Goal
Blank	Empty project	Minimal: just an "Overview" section with a prompt to get started
SaaS MVP	Basic section structure	Full starter PRD: Overview, User Personas, Core Features, Tech Stack, Data Model, API Design, Auth, Deployment, Timeline
Mobile App	Basic section structure	Mobile-focused: Overview, Platform Requirements, UI/UX, Offline Strategy, Push Notifications, App Store, Performance Targets
API Design	Basic section structure	API-focused: Overview, Endpoints, Auth, Rate Limiting, Error Handling, Versioning, SDK/Client Libraries, OpenAPI Spec

Acceptance Criteria

• Each template has thoughtful starter content (not just headings)
• Content includes guiding prompts/questions to help users fill in sections
• Section dependencies are pre-configured where obvious (e.g., Data Model → API Design)
• Templates are stored in a maintainable format (not hardcoded strings)
• New Project modal shows template previews (section count, description)

Notes

Separated from the UI redesign effort (branch redesign/stitch-ui). Can be implemented independently on main.