🛡️ Sentinel: Limit concurrent daemon socket connections to prevent DoS

[minibota]

Vulnerability:
The daemon's socket listener (startSocketListener in cmd/daemon.go) spawned a new goroutine for every incoming connection without any concurrency limit. This allowed a local attacker to cause a Denial of Service (DoS) by opening thousands of connections and keeping them idle, exhausting system resources (goroutines and memory).

Fix:
Implemented a semaphore pattern using a buffered channel (maxConcurrentConnections = 50). The listener now acquires a token before spawning a handler goroutine and releases it upon completion. This limits the number of active handlers to 50, providing backpressure to the OS socket backlog when the limit is reached.

Verification:
Added a new regression test TestConcurrencyLimit in cmd/security_concurrency_test.go. The test:

1. Overrides the limit to 10.
2. Spawns 50 concurrent client connections.
3. Verifies that the number of goroutines increases by ~60 (50 clients + 10 handlers) instead of ~100 (unbounded).

Risk Assessment:

• Impact: Prevents local resource exhaustion DoS.
• Regression Risk: Low. Legitimate traffic exceeding 50 concurrent active execution reports (which is very high for a single agent) will face delays (queued in OS backlog) or timeouts, which is the intended fail-safe behavior.
• Compatibility: No API changes.

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PR created automatically by Jules for task 9011405027792562287 started by @minibota

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