🛡️ Sentinel: [CRITICAL] Fix Zip Slip (path traversal) in skills import

[thirdeyenation]

🚨 Severity: CRITICAL
💡 Vulnerability: Zip Slip (Path Traversal) vulnerability in helpers/skills_import.py where zipfile.ZipFile.extractall() is called without path validation.
🎯 Impact: A maliciously crafted zip file could extract files outside the intended temporary directory, potentially overwriting sensitive system or application files.
🔧 Fix: Iterated through the archive members and explicitly verified that each member's resolved path is contained within the intended target directory using Path.is_relative_to() before extracting.
✅ Verification: Ran pytest tests/test_skills_runtime.py to ensure existing skill extraction functionality remains intact.

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PR created automatically by Jules for task 16453137044410793851 started by @thirdeyenation

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