feat(brand): App icon, palette and chat-to-computer motif

[showxu]

Summary

• Defines the Computer MCP App icon in Iconography: a vintage computer drawn as ivory Liquid Glass on a violet background, with an amber screen, vents, knob and a full-width chin inset behind the case. The layer sources live in Brand/app-icon/layers/ and the grid matches Xcode's App icon template.
• Derives the organization palette from the icon: violet tertiary, amber signal, ivory neutrals and the computer-* component tokens in DESIGN.md.
• Renders the App icon from those layers in Brand/render.mjs through the App icon generator (APP_ICONS_SCRIPT), and records the ictool version and generator digest in the export manifest.
• Draws the chat-to-computer motif in README headers and social cards: a chat request card, a secure link, and the flat computer with the member glyph or command result on its screen. Copy comes from Brand/config.json.
• Syncs the organization profile exports.

Checks

• python3 Brand/brand.py check: 56 brand exports match their manifest
• python3 Brand/brand.py sync . --repository .github: brand lock verified
• npm --prefix Brand run lint: 0 errors, 0 warnings
• Exports reviewed in light and dark, en and zh-CN