🛡️ Sentinel: [CRITICAL] Fix insecure random number generation

[thirdeyenation]

🚨 Severity: CRITICAL
💡 Vulnerability: Used random.choices instead of cryptographically secure secrets.choice for generating session IDs, root passwords, and agent IDs.
🎯 Impact: Attackers could potentially predict generated identifiers, leading to session hijacking, authentication bypass, or other security breaches.
🔧 Fix: Replaced random.choices with secrets.choice for secure pseudo-random number generation in agent.py, prepare.py, and helpers/guids.py.
✅ Verification: Ran syntax checks and pytest suite to ensure secrets module is used appropriately.

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PR created automatically by Jules for task 11582429185698668949 started by @thirdeyenation

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