🛡️ Sentinel: [MEDIUM] Fix CWE-276 file permission vulnerabilities

[rschumann]

🚨 Severity: MEDIUM
💡 Vulnerability: Cached discovery entries and model registries were written with 0644 permissions, which could expose sensitive data to other local users.
🎯 Impact: Unauthorized local users could potentially read cached API entries and model configurations.
🔧 Fix: Changed file creation permissions to 0600 in internal/discovery/cache.go and internal/intelligence/discovery/service.go to ensure only the owner can read/write the files.
✅ Verification: Run gosec ./... to verify the G306 (CWE-276) alerts are resolved for these files.

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PR created automatically by Jules for task 5791438759166116124 started by @rschumann

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