🛡️ Sentinel: Limit concurrent socket connections to prevent DoS

[minibota]

🔐 Issue: The daemon's Unix socket listener (startSocketListener) accepted an unlimited number of concurrent connections, spawning a new goroutine for each one. A local attacker could exploit this to exhaust system resources (Goroutines, memory), leading to a Denial of Service (DoS).

🧠 Risk: High. A malicious local user or a compromised process could crash the agent or make it unresponsive, preventing legitimate cron jobs from reporting their status or receiving updates.

🛠️ Fix: Introduced a semaphore (buffered channel) to limit the number of concurrent active socket handlers to 50 (maxConcurrentConnections).

• Connections are accepted by the listener loop.
• Before spawning a handler goroutine, the loop attempts to acquire a token from the semaphore.
• If the limit is reached, the loop blocks, providing backpressure. The OS socket backlog handles queuing pending connections.
• The token is released when the handler completes.

🧪 Tests: Added TestBoundedConcurrency in cmd/security_concurrency_test.go.

• This test verifies that the daemon correctly limits concurrency.
• It overrides the limit to 5, launches 50 clients, and asserts that the number of goroutines increases by ~5 (plus clients) rather than ~55.

📎 Notes: This PR was created by an automated nightly security task.

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PR created automatically by Jules for task 618719565753858636 started by @minibota

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