refactor: consolidate config logic into ConfigService

Created centralized ConfigService for configuration management.
Added deprecation comments to ConfigBuilder and AgentBuilder.
Platform-specific defaults centralized in one place.
Removed placeholder ConfigService from agent_lifecycle.go.

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

@@ -91,6 +91,7 @@ func NewAgentBuild(c *gin.Context) {
 }
 
 // UpgradeAgentBuild handles agent upgrade requests
+// Deprecated: Use ConfigService for config building
 func UpgradeAgentBuild(c *gin.Context) {
 	agentID := c.Param("agentID")
 	if agentID == "" {