update project structure

- move agent to /agent
- change /beszel to /src
- update workflows and docker builds

agent/agent_test_helpers.go

@@ -0,0 +1,9 @@
+//go:build testing
+// +build testing
+
+package agent
+
+// TESTING ONLY: GetConnectionManager is a helper function to get the connection manager for testing.
+func (a *Agent) GetConnectionManager() *ConnectionManager {
+	return a.connectionManager
+}