update project structure

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

agent/battery/battery_freebsd.go

@@ -0,0 +1,13 @@
+//go:build freebsd
+
+package battery
+
+import "errors"
+
+func HasReadableBattery() bool {
+	return false
+}
+
+func GetBatteryStats() (uint8, uint8, error) {
+	return 0, 0, errors.ErrUnsupported
+}