Refactor loop to main and add explicit while(true)

This change transitions from the Arduino-specific `loop()` to a
standard C++ `main()` function as the program's entry point and
primary execution cycle.

src/atmega/main.cpp

@@ -31,7 +31,8 @@ void setup(void) {
     watchdogs();
 }
 
-void loop(void) {
+int main(void) {
+    while(true) {
         if (readSensors) {
             int16_t result = readMoisture();
             if (result != -1) {
@@ -39,6 +40,7 @@ void loop(void) {
                 readSensors = false;
             }
         }
-
         enterSleep();
     }
+    return 0;
+}