fix: prevent path-traversal sandbox escape in filesystem operations

[hashemix]

📌 Summary

Fix path-traversal sandbox escape in filesystem operations. The normalize_path function fell back to raw (unsanitized) paths when canonicalize() failed for non-existent files, allowing .. components to bypass validate_path. This enables escaping the allowed directory sandbox via write_file, create_directory, move_file, and edit save-to.

🔍 Related Issues

✨ Changes Made

• normalize_path now attempts to canonicalize the parent directory and rejoin the file name when the full path doesn't exist. Falls back to raw path only as last resort.
• validate_path returns the normalized path instead of the raw absolute_path. Added defense-in-depth rejection of any path still containing Component::ParentDir after normalization.
• save_to now passes through validate_path with the allowed directories set (previously skipped validation entirely).
• Added 10 regression tests covering .. escape, symlink escape (unix), write/create/move/edit end-to-end escapes, multi-level .., resolved-in-root paths, non-existent-with-out-of-bounds-parent, and defense-in-depth.

🛠️ Testing Steps

cargo make check

💡 Additional Notes

• A GitHub Security Advisory (with CVE request) has been drafted and will be published simultaneously with the release containing this fix.
• SECURITY.md added to document the vulnerability reporting process.
• CVE ID to be added to CHANGELOG.md at release time.