connect-gridlines-to-page-edges.md

@primer/brand-mcp	patch

Updated MCP gridline tools to prevent disconnected, centered border boxes:

• Updated primer_brand_page_design to prefer built-in gridline APIs and scope custom page-width frames to grouped Cards and Pillars.
• Updated primer_brand_examples to preserve canonical Card/Pillar companion CSS and surface the focused Statistic gridline example.