🛡️ Sentinel: [HIGH] Fix path traversal in LocalJsonStore

[hackerxj2010]

🚨 Severity: HIGH
💡 Vulnerability: Path traversal in LocalJsonStore class within the @jeanbot/documents package.
🎯 Impact: An attacker providing a malicious key (e.g., ../../etc/passwd) could potentially read or write files outside the intended base directory.
🔧 Fix:

• Modified LocalJsonStore.toPath to reject absolute paths and paths containing . or .. segments.
• Implemented canonicalization using path.resolve.
• Added a prefix check against the resolved base directory with a trailing separator to prevent prefix bypass attacks.
  ✅ Verification:
• Added comprehensive security tests in packages/documents/src/security.test.ts covering absolute paths, relative traversal, encoded sequences, and Windows-style separators.
• Ran pnpm test from the root, verifying all 52 tests across the monorepo pass.
• Verified pnpm lint passes for the modified package.
• Created security journal in .jules/sentinel.md.

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PR created automatically by Jules for task 15927695176905117538 started by @hackerxj2010

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