docs: document AGENTS.md and CLAUDE.md in create-next-app CLI docs
- Update --yes flag description to mention AGENTS.md and CLAUDE.md
- Update recommended defaults prompt to show AGENTS.md (matching CLI output)
- Add AGENTS.md prompt to customize settings section
- Add --agents-md flag to create-next-app API reference options table
- Apply changes to both installation and create-next-app reference pages
Co-authored-by: Joseph <icyJoseph@users.noreply.github.com>