feat: Establish functional multi-app structure

This commit captures a working multi-app build where the board definition is located in the 'software' directory and explicitly included by the slave_node application. This serves as a stable baseline.

software/apps/slave_node/src/main.c

@@ -8,6 +8,9 @@
 
 int main(void)
 {
-	printk("Hello from Slave Node!\n");
+	while (1) {
+		printk("Hello from Slave Node!\n");
+		k_msleep(1000);
+	}
 	return 0;
 }