Add AGENTS.md with development environment setup instructions

[sarahkanee]

Summary

Adds AGENTS.md with Cursor Cloud specific instructions for setting up and running the R-based ecological forecasting pipeline.

What's included

• Project overview describing the EE 585 aquatic forecasting pipeline
• Required R packages table (tidyverse, lubridate, rMR v1.1.0, neon4cast, arrow)
• Instructions for running the pipeline (Rscript main.R)
• Known issues documented (R Markdown syntax in 02_calibrate_forecast.R and 02_calibrate_forecast_Eric.R)
• Notes about external data dependencies (NEON targets, NOAA GEFS)

Environment verification

All 4 required packages load and execute correctly:

r_environment_demo.log

_{To show artifacts inline, enable in settings.}