docs: improve README documentation

[jevin98]

What changed

• Rewrote the English README to better explain CSS-Diff's purpose, strengths, features, installation, usage, development, build commands, tech stack, inspiration, and license.
• Added a Simplified Chinese README with matching structure and reciprocal language links.
• Added README guidance to AGENTS.md so future README generation keeps all language versions included and synchronized.

Why

The previous README only had a short description and minimal usage notes, so it did not clearly communicate the extension's value for frontend debugging or its core workflow.

Impact

Users and contributors get a clearer project overview in both English and Simplified Chinese, with accurate documentation for the current DevTools comparison behavior.

Validation

• pnpm install --frozen-lockfile
• pnpm exec eslint . --fix
• git diff --check
• pnpm compile

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Document unpacking the release zip before loading

In the newly added Chinese README, users are told to download the packaged zip and then manually install/load it from the extensions page, but Chrome/Edge's unpacked-extension flow cannot load the .zip artifact directly; it needs to be extracted first and the output directory selected with "Load unpacked". Chinese users following these install steps will be blocked before they can try the extension, so this should explicitly mention unzipping or provide browser-specific package instructions.

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