🛡️ Sentinel: [CRITICAL] Fix Zip Slip vulnerability in skills import

[thirdeyenation]

🚨 Severity: CRITICAL
💡 Vulnerability: _unzip_to_temp_dir in helpers/skills_import.py uses z.extractall(target) without verifying member paths, leading to a Zip Slip vulnerability.
🎯 Impact: A malicious skill ZIP could overwrite critical system or application files by using directory traversal sequences (../) or absolute paths in the archive.
🔧 Fix: Added explicit member path validation. It resolves the intended extraction path for each member and uses is_relative_to to ensure the member falls safely within the target extraction directory before calling extractall.
✅ Verification: Verified using an isolated python script generating a malicious zip and confirming a ValueError is raised instead of extracting the malicious file.

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PR created automatically by Jules for task 16947130442591053172 started by @thirdeyenation

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