[Design] Add brand design system presets (Stripe, Vercel, Linear style)

[jeromwolf]

Description

We now have 54 brand DESIGN.md files synced from awesome-design-md!

Add preset support so users can generate pages matching famous brands:

• /templateclaw-landing saas --brand stripe → Stripe's purple + blue-tinted shadows
• /templateclaw-landing saas --brand vercel → Vercel's black + white minimal
• /templateclaw-landing saas --brand linear → Linear's purple gradients

Available Brands

stripe, vercel, linear, claude, notion, supabase, figma, cursor, spotify, airbnb, apple + 43 more

How

Read designs/{brand}/DESIGN.md and extract color palette, typography, and component styles.