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/sensors.cpp

@@ -0,0 +1,22 @@
+#include <Adafruit_BMP085.h>
+#include <math.h>
+#include "sensors.h"
+
+static Adafruit_BMP085 bmp;
+
+bool sensorsBegin() {
+  return bmp.begin();          // uses Wire (already started on D6/D1), addr 0x77
+}
+
+float readAbsPressureHpa() {
+  return bmp.readPressure() / 100.0f;   // Pa -> hPa
+}
+
+float readTemperatureC() {
+  return bmp.readTemperature();
+}
+
+// Standard barometric reduction to sea level.
+float toSeaLevelHpa(float absHpa, float altitudeM) {
+  return absHpa / powf(1.0f - (altitudeM / 44330.0f), 5.255f);
+}