docs(readme): rewrite in the nanobot register — start-here table, short sections, Android download, absolute links

Cover with the plush avatar, badges, one-line tagline, "What can OpenMuse do?",
emoji section headers, an install table, a three-command quick start, a 📱 Android
section with the fixed openmuse.apk link, Deploy, the app in one table, Sentinel and
Models kept short, Releases and Recent Updates by version. Image and doc links are
absolute so PyPI renders them too. Size numbers updated (18k Python, 11k TypeScript,
800 Kotlin). Screenshots of the old look removed; the Chinese README mirrors it.

Author: lgy0404