Improve GEE setup guidance and issue intake

[peetmate]

Summary

• move Earth Engine setup earlier in README and strengthen placeholder guidance
• add README-linked GEE auth/project error messaging
• add GitHub issue templates and label guidance
• add demo helper script for issue Source_data module is not working #69 repro

Testing

• .venv/bin/python -m pytest -q tests/test_xee_common.py tests/test_fetch_pipeline.py
• .venv/bin/python -m pytest -q tests/test_vpd.py

Related

• Improve Earth Engine authentication and project setup UX #84
• Expose bounded worker controls for historical GEE/Xee CLI #85
• Clip out-of-coverage GEE date requests with clear warning and continue #86