route-agents-to-primer-brand.md

@primer/brand-mcp	minor

Updated primer_brand_setup to create or update AGENTS.md beside a package.json that declares @primer/react-brand.

• Preserves existing instructions and appends/replaces only the Primer Brand instructions.
• Requires projectDir when several workspace packages declare Primer Brand.
• Routes future agents to the MCP tools and version-matched local documentation.