[codex] Add AgentKeys logo assets

[hanwencheng]

Summary

• add the supplied AgentKeys logo as canonical brand assets under docs/assets/brand
• generate transparent light-theme and dark-theme logo sets, favicon PNGs, favicon.ico variants, touch icons, and web app icons
• simplify the lower ear contours from the original wavy bottoms into single rounded arcs for a cleaner, cuter mark
• wire the README logo through prefers-color-scheme so the black mark is used on light surfaces and the white mark is used on dark surfaces
• include a minimal web manifest and brand README usage snippets

Validation

• python3 -m json.tool docs/assets/brand/site.webmanifest
• verified generated PNG dimensions, transparent corners, and theme colors with Pillow
• visually checked the simplified-ear light and dark logo variants plus favicon previews