chore: make repo an Arduino collection with WeatherPredictor project

Flatten so the repo root is the Arduino sketchbook; the project lives in
WeatherPredictor/. Add a collection index README at the root and move the
detailed project README into WeatherPredictor/.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

WeatherPredictor/rtc_time.h

@@ -0,0 +1,14 @@
+#pragma once
+#include <Arduino.h>
+
+struct RtcTime {
+  uint16_t year;
+  uint8_t  month, day, hour, minute, second;
+};
+
+bool     rtcBegin();
+bool     rtcLostPower();
+RtcTime  rtcNow();
+void     rtcSet(const RtcTime& t);
+uint32_t rtcEpoch();
+bool     ntpSync(int tzOffsetMin);   // true if synced and RTC updated