FIFO semaphore that limits the number of concurrent async operations.

When the concurrency limit is reached, subsequent acquire() calls queue up and wait for a slot to become available. Each successful acquire returns a release function that must be called when the operation completes to free the slot. The release function is idempotent — calling it multiple times has no effect.

Queue entries can time out via the timeoutMs parameter, throwing a SemaphoreTimeoutError. Used by AgentRuntime to limit concurrent tool executions via the maxConcurrentToolExecutions config.

class Semaphore {
    constructor(limit: number)
    acquire(timeoutMs: number, timeoutCode:  = 'SEMAPHORE_QUEUE_TIMEOUT', timeoutMessage:  = 'Semaphore acquire timeout') => Promise<() => void>;
    getInUseCount() => number;
    getQueueLength() => number;
}