docs: improve README clarity and examples list

[meichuanyi]

Summary

• Clarify the opening description to avoid repeating "sandbox" and fix the typo "exfilteration" -> "exfiltration"
• Align the Examples section with the actual directory names under examples/
• Add the existing gemini_cli example to the README list

Testing

• Documentation-only change; not run

[gemini-code-assist]

Code Review

This pull request updates the README.md file to improve the wording of the project description, correct a typo ('exfilteration' to 'exfiltration'), rename example directories to use underscores, and document a new Gemini CLI example. As there are no review comments, no further feedback is provided.

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