🛡️ Sentinel: Fix path traversal in LocalJsonStore

[hackerxj2010]

🚨 Severity: CRITICAL
💡 Vulnerability: Path traversal in LocalJsonStore
🎯 Impact: Attackers could read or write arbitrary JSON files on the filesystem outside the intended base directory.
🔧 Fix: Implemented multiple layers of protection:

1. Disallow absolute paths in keys.
2. Filter out . and .. segments.
3. Resolve the final path and verify it starts with the baseDirectory (using a trailing separator to prevent prefix bypass).
   ✅ Verification: Added packages/documents/src/security.test.ts which confirms that traversal attempts are blocked.

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PR created automatically by Jules for task 6235994778132024140 started by @hackerxj2010

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