Add polygon region examples and python-script-writer agent docs

[Copilot]

Adds two new rips workflow examples for polygon-based grid region assignment, plus an agent instruction document to guide future Python script authoring.

New examples

• polygon_grid_region.py — creates N polygons programmatically from bounding-box coordinates, adds them to the project's PolygonCollection, then writes a GENERATED property POLYGON_REGION assigning each active cell a 1-based region index via 2D ray-casting.

• polygon_region_from_project.py — reads polygons already present in the project and applies the same region-assignment logic:

polygon_collection = resinsight.project.descendants(rips.PolygonCollection)[0]
polygons = polygon_collection.polygons()
# ...
case.set_active_cell_property(region_values, "GENERATED", "POLYGON_REGION", 0)
view.apply_cell_result(result_type="GENERATED", result_variable="POLYGON_REGION")

Both examples are wired into PythonExamples_case_and_grid_operations.rst.

Agent instructions

• agents/python-script-writer.md — describes script skeleton, error-guard conventions, polygon patterns (create vs. read), cell property I/O, descendants() navigation, and style rules. References https://api.resinsight.org throughout.
• CLAUDE.md — updated to advertise the new agent alongside the existing docs-writer.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.