🛡️ Sentinel: [CRITICAL] Fix path traversal in LocalJsonStore

[hackerxj2010]

🛡️ Sentinel: [CRITICAL] Fix path traversal in LocalJsonStore

This change secures the LocalJsonStore against path traversal vulnerabilities by:

1. Filtering out . and .. segments from provided keys.
2. Using path.relative to verify that the final resolved path remains within the store's base directory.

A new security test suite has been added to verify these protections and prevent regressions.

🚨 Severity: CRITICAL
💡 Vulnerability: Path traversal in LocalJsonStore
🎯 Impact: Unauthorized file read/write outside the designated storage directory.
🔧 Fix: Key sanitization and boundary validation using path.relative.
✅ Verification: Ran packages/documents/src/security.test.ts and the full pnpm test suite.

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PR created automatically by Jules for task 7769651726634502811 started by @hackerxj2010

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