🛡️ Sentinel: [MEDIUM] Fix fd leak and unsafe path bridging on PID lock

[acebytes]

🚨 Severity: MEDIUM
💡 Vulnerability: Missing O_CLOEXEC causes the file descriptor to leak to child processes. Implicit path string bridging may not produce a valid file system representation.
🎯 Impact: Child processes could inherit the PID lock file descriptor, preventing the lock from being released on daemon shutdown.
🔧 Fix: Added O_CLOEXEC, used S_IRUSR | S_IWUSR for atomic 0600 mode application, and withUnsafeFileSystemRepresentation for safe string passing.
✅ Verification: Verified file modifications and executed test suite successfully.

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PR created automatically by Jules for task 16921181005958822429 started by @acebytes

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