Add security disclaimer to README

[Copilot]

Added prominent security disclaimer to communicate that no isolation mechanism is foolproof and users bear sole responsibility for executing malicious code.

Changes

• Added "⚠️ Security Disclaimer" section after package description in README.md
• States that process isolation, Docker, Firejail, and macOS sandboxes reduce but don't eliminate risk
• Explicitly assigns liability to users for consequences of running untrusted code
• Positioned before Installation to ensure visibility

The disclaimer addresses the gap between the package's stated purpose (executing untrusted code) and the lack of clear communication about security limitations.

Original prompt

Add a disclaimer to the top of the readme on the limitations of security and that it is the users own responsibility if malicious code is run.

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