Add Mermaid fullscreen mode

[benvinegar]

Summary

• add a Mermaid-only fullscreen modal for large diagrams, preserving sandboxed iframe rendering
• add keyboard/accessibility handling for the modal: focus entry, Escape close, Tab wrapping, and focus restoration
• update agent guides to prefer vertical/card-friendly Mermaid layouts and split oversized architecture maps

Validation

• npm run typecheck
• npm run lint
• npm run format:check
• npm test
• npm run build
• subagent code review: no blockers

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