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/client/client.go

@@ -174,8 +174,9 @@ type Command struct {
 
 // CommandsResponse contains pending commands
 type CommandsResponse struct {
-	Commands     []Command           `json:"commands"`
-	RapidPolling *RapidPollingConfig `json:"rapid_polling,omitempty"`
+	Commands          []Command           `json:"commands"`
+	RapidPolling      *RapidPollingConfig `json:"rapid_polling,omitempty"`
+	AcknowledgedIDs   []string            `json:"acknowledged_ids,omitempty"` // IDs server has received
 }
 
 // RapidPollingConfig contains rapid polling configuration from server
@@ -196,11 +197,15 @@ type SystemMetrics struct {
 	Uptime        string                    `json:"uptime,omitempty"`
 	Version       string                    `json:"version,omitempty"`        // Agent version
 	Metadata      map[string]interface{} `json:"metadata,omitempty"`      // Additional metadata
+
+	// Command acknowledgment tracking
+	PendingAcknowledgments []string `json:"pending_acknowledgments,omitempty"` // Command IDs awaiting ACK
 }
 
 // GetCommands retrieves pending commands from the server
 // Optionally sends lightweight system metrics in the request
-func (c *Client) GetCommands(agentID uuid.UUID, metrics *SystemMetrics) ([]Command, error) {
+// Returns the full response including commands and acknowledged IDs
+func (c *Client) GetCommands(agentID uuid.UUID, metrics *SystemMetrics) (*CommandsResponse, error) {
 	url := fmt.Sprintf("%s/api/v1/agents/%s/commands", c.baseURL, agentID)
 
 	var req *http.Request
@@ -252,7 +257,7 @@ func (c *Client) GetCommands(agentID uuid.UUID, metrics *SystemMetrics) ([]Comma
 		}
 	}
 
-	return result.Commands, nil
+	return &result, nil
 }
 
 // UpdateReport represents discovered updates