🛡️ Sentinel: [MEDIUM] Fix predictable temporary file generation

[Kaos599]

🚨 Severity: MEDIUM
💡 Vulnerability: The atomicWriteFile and writeFileAtomic functions used Math.random() to generate temporary suffixes when writing files. Since Math.random() is not cryptographically secure, it could theoretically allow an attacker with local access to predict temporary file names and attempt symlink or race condition attacks to intercept or modify sensitive files like agent configurations and backups.
🎯 Impact: Attackers on the same system might predict file names to mount symlink attacks or intercept configuration writes.
🔧 Fix: Replaced Math.random().toString(36) and Math.random().toString(16) with crypto.randomBytes(8).toString("hex") to ensure generated filenames are unpredictable and cryptographically secure.
✅ Verification: Ran pnpm test successfully. Confirmed crypto.randomBytes is used in both src/fs-utils.ts and src/backup/archive.ts.

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PR created automatically by Jules for task 18397922664517755359 started by @Kaos599

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