fix: terminal sessions auto-heal stale ArcGISProTemp scratch workspace

[danmaps]

Closes #4.

What

• Terminal launcher now writes a small .arcgispro/sitecustomize.py hook into the project.
• Terminal launcher sets:
  • ARCGISPRO_PROJECT_DIR (quoted)
  • PYTHONPATH to include <project>\.arcgispro so sitecustomize.py auto-runs for any Python process started from the terminal.

Why

ArcGIS Pro creates session temp dirs like ArcGISProTemp<PID>. When those get cleaned up, stale arcpy.env.workspace / scratchWorkspace references can cause:
Directory does not exist or cannot be accessed: ...ArcGISProTemp####.

How it works

On Python startup (only when launched from the Pro terminal):

• If arcpy isn't available, do nothing.
• If workspace or scratchWorkspace points at a missing ArcGISProTemp* path, clear it.
• Prefer <project>/scratch.gdb (create if missing) as a stable scratch/workspace.

Notes

• Hook is defensive + silent; it should never crash Python startup or block user scripts.