refactor: add AgentLifecycleService for unified agent operations

Created centralized lifecycle service to handle new, upgrade, and rebuild operations.
Added deprecation notices to old handlers (agent_setup, build_orchestrator, agent_build).
Foundation for consolidating duplicate agent lifecycle logic.

aggregator-server/internal/api/handlers/build_orchestrator.go

@@ -9,6 +9,7 @@ import (
 )
 
 // NewAgentBuild handles new agent installation requests
+// Deprecated: Use AgentHandler.Upgrade instead
 func NewAgentBuild(c *gin.Context) {
 	var req services.NewBuildRequest
 	if err := c.ShouldBindJSON(&req); err != nil {