feat(gitagent): add "Use Cases" section to the landing page

[Nivesh353]

What

Adds a Use Cases section ("What you can build") to the GitAgent landing
page (/), placed after Features. It presents five representative agents you
can build with GitAgent, plus a sixth "And more" tile linking to the cookbooks.

Cards

• Code Review Agent
• Personal Assistant with Memory
• Customer Support Agent
• Voice Assistant
• Automated Research & Report Writer
• "And more" (example chips + cookbooks link)

Files

• src/components/gitAgent/GitAgentUseCasesSection.tsx (new)
• src/pages/GitAgentPage.tsx — render + sticky page-nav entry
• Renumbered eyebrow labels in Models / Memory / Integrations / Install
  sections (06→07 … 09→10) to keep the sequence intact

Notes

• Content-only / presentational — no backend or SDK dependency, safe to merge
  independently.

[vercel]

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