feat(tui): ContributingViewer fragment with markdown rendering

[cdhagmann]

Renders the project's CONTRIBUTING.md per ADR-0007 (display, don't parse). May absorb #13 (PR template surfacing) if implementation can handle both.

Scope:

• GemContribute::TUI::ContributingViewer Rooibos fragment
• Markdown rendering — pick the smallest renderer that handles headings, lists, code blocks, links
• Coordinate with Surface upstream PR templates in the CONTRIBUTING viewer #13 (PR template surfacing) — likely the same fragment with section navigation
• Pure-function Update test

Sub-issue of #2.

See: ADR-0007, #13, docs/design.md TUI section, ROADMAP Phase 3