feat: add resilience and reliability features for agent subsystems

Added circuit breakers with configurable timeouts for all subsystems (APT, DNF, Docker, Windows, Winget, Storage). Replaces cron-based scheduler with priority queue that should scale beyond 1000+ agents if your homelab is that big.

Command acknowledgment system ensures results aren't lost on network failures or restarts. Agent tracks pending acknowledgments with persistent state and automatic retry.

- Circuit breakers: 3 failures in 1min opens circuit, 30s cooldown
- Per-subsystem timeouts: 30s-10min depending on scanner
- Priority queue scheduler: O(log n), worker pool, jitter, backpressure
- Acknowledgments: at-least-once delivery, max 10 retries over 24h
- All tests passing (26/26)

aggregator-agent/internal/circuitbreaker/circuitbreaker_test.go

@@ -0,0 +1,138 @@
+package circuitbreaker
+
+import (
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestCircuitBreaker_NormalOperation(t *testing.T) {
+	cb := New("test", Config{
+		FailureThreshold: 3,
+		FailureWindow:    1 * time.Minute,
+		OpenDuration:     1 * time.Minute,
+		HalfOpenAttempts: 2,
+	})
+
+	// Should allow calls in closed state
+	err := cb.Call(func() error { return nil })
+	if err != nil {
+		t.Fatalf("expected no error, got %v", err)
+	}
+
+	if cb.State() != StateClosed {
+		t.Fatalf("expected state closed, got %v", cb.State())
+	}
+}
+
+func TestCircuitBreaker_OpensAfterFailures(t *testing.T) {
+	cb := New("test", Config{
+		FailureThreshold: 3,
+		FailureWindow:    1 * time.Minute,
+		OpenDuration:     100 * time.Millisecond,
+		HalfOpenAttempts: 2,
+	})
+
+	testErr := errors.New("test error")
+
+	// Record 3 failures
+	for i := 0; i < 3; i++ {
+		cb.Call(func() error { return testErr })
+	}
+
+	// Should now be open
+	if cb.State() != StateOpen {
+		t.Fatalf("expected state open after %d failures, got %v", 3, cb.State())
+	}
+
+	// Next call should fail fast
+	err := cb.Call(func() error { return nil })
+	if err == nil {
+		t.Fatal("expected circuit breaker to reject call, but it succeeded")
+	}
+}
+
+func TestCircuitBreaker_HalfOpenRecovery(t *testing.T) {
+	cb := New("test", Config{
+		FailureThreshold: 2,
+		FailureWindow:    1 * time.Minute,
+		OpenDuration:     50 * time.Millisecond,
+		HalfOpenAttempts: 2,
+	})
+
+	testErr := errors.New("test error")
+
+	// Open the circuit
+	cb.Call(func() error { return testErr })
+	cb.Call(func() error { return testErr })
+
+	if cb.State() != StateOpen {
+		t.Fatal("circuit should be open")
+	}
+
+	// Wait for open duration
+	time.Sleep(60 * time.Millisecond)
+
+	// Should transition to half-open and allow call
+	err := cb.Call(func() error { return nil })
+	if err != nil {
+		t.Fatalf("expected call to succeed in half-open state, got %v", err)
+	}
+
+	if cb.State() != StateHalfOpen {
+		t.Fatalf("expected half-open state, got %v", cb.State())
+	}
+
+	// One more success should close it
+	cb.Call(func() error { return nil })
+
+	if cb.State() != StateClosed {
+		t.Fatalf("expected closed state after %d successes, got %v", 2, cb.State())
+	}
+}
+
+func TestCircuitBreaker_HalfOpenFailure(t *testing.T) {
+	cb := New("test", Config{
+		FailureThreshold: 2,
+		FailureWindow:    1 * time.Minute,
+		OpenDuration:     50 * time.Millisecond,
+		HalfOpenAttempts: 2,
+	})
+
+	testErr := errors.New("test error")
+
+	// Open the circuit
+	cb.Call(func() error { return testErr })
+	cb.Call(func() error { return testErr })
+
+	// Wait and attempt in half-open
+	time.Sleep(60 * time.Millisecond)
+	cb.Call(func() error { return nil }) // Half-open
+
+	// Fail in half-open - should go back to open
+	cb.Call(func() error { return testErr })
+
+	if cb.State() != StateOpen {
+		t.Fatalf("expected open state after half-open failure, got %v", cb.State())
+	}
+}
+
+func TestCircuitBreaker_Stats(t *testing.T) {
+	cb := New("test-subsystem", Config{
+		FailureThreshold: 3,
+		FailureWindow:    1 * time.Minute,
+		OpenDuration:     1 * time.Minute,
+		HalfOpenAttempts: 2,
+	})
+
+	stats := cb.GetStats()
+	if stats.Name != "test-subsystem" {
+		t.Fatalf("expected name 'test-subsystem', got %s", stats.Name)
+	}
+	if stats.State != "closed" {
+		t.Fatalf("expected state 'closed', got %s", stats.State)
+	}
+	if stats.RecentFailures != 0 {
+		t.Fatalf("expected 0 failures, got %d", stats.RecentFailures)
+	}
+}