Document design system guidelines and UI implementation requirements

[Parsh]

This pull request updates the project configuration to formalize the use of a canonical UI design system and strengthen UI implementation guidelines. The main focus is on ensuring that all UI work adheres to the standards set in DESIGN.md, and that contributors reference and follow these standards in their workflow.

Design System and UI Implementation Guidelines:

• Added a new section to openspec/config.yaml that establishes DESIGN.md as the canonical source for UI design tokens, layout patterns, component reuse, and security UX principles. It requires all contributors to read and follow DESIGN.md before making UI changes, and clarifies the relationship between the codebase and the design document.
• Updated the pull request checklist to require references to relevant design tokens, patterns, and existing components from DESIGN.md for any UI-related change. Contributors must also justify any new components and confirm they've completed the implementation checklist in DESIGN.md before finalizing UI design.…ments in config.yaml