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-server/internal/models/command.go

@@ -23,8 +23,9 @@ type AgentCommand struct {
 
 // CommandsResponse is returned when an agent checks in for commands
 type CommandsResponse struct {
-	Commands       []CommandItem `json:"commands"`
-	RapidPolling   *RapidPollingConfig `json:"rapid_polling,omitempty"`
+	Commands        []CommandItem        `json:"commands"`
+	RapidPolling    *RapidPollingConfig  `json:"rapid_polling,omitempty"`
+	AcknowledgedIDs []string             `json:"acknowledged_ids,omitempty"` // IDs server has received
 }
 
 // RapidPollingConfig contains rapid polling configuration for the agent