contributing: Added copilot agent instructions

[cwhite911]

Here are basic instructions for the Copilot. These stemmed from an early experiment for a FOSS4GNA workshop this past fall.

The instructions were mostly base off the the GRASS Programming Style Guide and initially designed to help developers convert GRASS scripts into GRASS addons following community best practices.

I've added this here now for visibility and additional discussion as @wenzeslaus is working on the Claude code agent in PR #7110

The instructions live here

.github/
├── copilot-instructions.md
└── instructions/
├──── c.instructions.md
├──── docs.instructions.md
├──── general.instructions.md
├──── pre-commit.instructions.md
├──── python.instructions.md
└──── requirements.instructions.md

[wenzeslaus]

I can't really tell if I already commented elsewhere mentioning this or never hit sent on a comment, but my quick eval here is that a lot of this does not make sense for the this repo. It made sense for a one-time workshop repo based on this repo, but here it would cause duplication and unclear maintenance procedures - prime example of that is the list of dependencies which is basically straight copy of the requirements file.

I suggest simplifying this to a minimal version which is probably link from .github/copilot-instructions.md to AGENTS.md and then whatever Copilot suggests for itself.