Update docs and install sheet component

[kevinrutledge]

What needs to be built?

Update documentation to reflect current team structure (no pairing) and technical decisions (32-bit signature). Install the sheet component for mobile navigation.

Documentation

Existing patterns:

• docs/getting-started.md — uses root-relative links
• src/components/ui/drawer.tsx — similar slide-in component

Official docs:

• shadcn/ui Sheet

Technical notes

• Link in docs/contributing.md line 7 needs root-relative path
• "Working in Pairs" section is lines 85-109 in docs/contributing.md
• Signature description is line 79 in docs/architecture.md
• Sheet slides from side (nav menus), Drawer slides from bottom (action sheets)

Acceptance criteria

• Broken link in docs/contributing.md fixed
• "Working in Pairs" section removed from docs/contributing.md
• Signature description in docs/architecture.md says "32-bit HMAC"
• Sheet component installed in src/components/ui/
• npm run lint passes
• npm run build passes