Add Build Canada constitution page

[xrendan]

Summary

• add a new /constitution page with Build Canada’s mission and seven guiding principles
• match the existing editorial design system with responsive, accessible markup
• include the page in the site sitemap

Validation

• pnpm exec eslint src/app/constitution/page.tsx src/app/sitemap.ts
• pnpm exec tsc --noEmit
• visually verified at desktop and mobile widths

Note: the full production build could not run concurrently with the active Next.js development server because it owns .next/lock.