🛡️ Sentinel: [MEDIUM] Enhance security for exported requests

[tejjasdev]

🚨 Severity: MEDIUM
💡 Vulnerability: Saved HTTP requests were stored in directories created with 0o755 permissions, meaning other users on the same machine could read the saved request files. In addition, the Set-Cookie header was not redacted by default.
🎯 Impact: This could potentially lead to information disclosure if a sensitive request (with tokens or sensitive payloads) is saved locally on a shared machine.
🔧 Fix:

• Changed os.MkdirAll permission from 0o755 to 0o700 in internal/export/export.go.
• Added Set-Cookie to DefaultRedactHeaders.
  ✅ Verification: make test passes successfully, and review shows standard library features used appropriately for securing the exported files.

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PR created automatically by Jules for task 8046021840658410554 started by @tejjasdev

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