🛡️ Sentinel: [CRITICAL] Fix weak random number generation

[thirdeyenation]

🚨 Severity: CRITICAL
💡 Vulnerability: Used random (deterministic PRNG) instead of secrets (cryptographically secure PRNG) for generating root passwords, agent IDs, and scheduler tokens.
🎯 Impact: Attackers could potentially predict generated identifiers or tokens, bypassing security controls.
🔧 Fix: Replaced random.choices with secrets.choice and random.randint with secrets.randbelow in prepare.py, agent.py, helpers/guids.py, api/scheduler_task_create.py, and helpers/task_scheduler.py.
✅ Verification: Ran pytest suite and compiled all changed Python files. Checked logs locally to ensure generation works properly.

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PR created automatically by Jules for task 12241692746722871186 started by @thirdeyenation

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