docs: add complete English README

[PuppetWen]

What changed

• Replaced the default README with a complete English guide for CDriveShiftAI 0.0.6.
• Documented download options, search behavior, content indexing, ownership analysis, safe migration, themes, shortcuts, storage, updates, performance, development, and safety boundaries.
• Added direct language navigation from the English README to the Chinese guide.
• Added a matching English link and documentation shortcuts to the Chinese guide.

Why

The repository's default README was still Chinese-only and contained older three-theme wording. International users could not review the current feature set or navigate cleanly between the English and Chinese documentation.

User impact

GitHub visitors can now open the default English documentation and switch between English and Simplified Chinese from the top of either README.

Validation

• git diff --check
• Verified every relative Markdown link points to an existing repository file.
• Verified the documented version and release artifact names against package.json and release-ready.