🛡️ Sentinel: Limit concurrent socket connections to prevent DoS

[minibota]

🔐 Issue: The daemon's socket listener (Accept loop) spawned a new goroutine for every incoming connection without any concurrency limit. This allowed a local attacker (or malfunctioning client) to flood the daemon with connections, leading to unbounded goroutine creation and potential resource exhaustion (DoS).

🧠 Risk: High. A simple loop of net.Dial could cause the daemon to consume all available memory or thrash the scheduler, crashing the security agent and stopping cron job synchronization/reporting.

🛠️ Fix:

• Introduced a maxConcurrentConnections limit (default: 100) using a semaphore pattern (buffered channel).
• The Accept loop now blocks when the limit is reached, creating backpressure on the OS socket backlog.
• Added a check for "use of closed network connection" error to suppress logs during graceful shutdown.

🧪 Tests:

• Added cmd/security_concurrency_test.go: TestBoundedConcurrency.
• This test verifies that even when maxConcurrentConnections is set to 5 and 20 clients connect, the number of active goroutines remains bounded (baseline + limit).
• Verified that the test fails without the fix and passes with it.
• Ran all existing tests to ensure no regressions.

📎 Notes: The default limit of 100 is conservative but sufficient for this agent's purpose (reporting job execution).

This PR was created by an automated nightly security task.

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PR created automatically by Jules for task 6996260778888758772 started by @minibota

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