[wenzeslaus]

Using a use case of a standalone addon tool repo (specifically, r.pops.spread is published in addons, but the developed in a standalone repo), this expands instructions in AGENTS.md on how to compile and run with CMake in a sandbox (developed with Claude Code sandbox) and how to compile an addon tool. While grass-addon and cookie cutter repo instructions would be helpful as well, the addons instruction part covers the case when you use the source code of GRASS, but don't deal with the grass-addon repo.

I'm also slighltly changing how the grass.gunittest is presented, focusing on a single file, rather than a directory run. This simplifies the project-related instructions, replacing the currently confusing version.

The addons piece is something which may possible confuse an agent (if you don't work with addons or g.extension), but I would just wait to see how it behaves.

@nilason or @HuidaeCho if you can check if the CMake prefix instructions are good, that would be great. This is a result based on how it behaved and what Claude came up with as an evaluation, but it is intended? I'm still not 100% comfortable with CMake, so it is hard for me to be sure this right and does have some red flags for me, mainly the use of -D vs --prefix.

[nilason]

Some of the code hard codes CMAKE_INSTALL_PREFIX e.g. during configuration of resource_paths.py.

cmake --install build --prefix ./tmp_install_dir is on the other hand, used for e.g. staging installation. This may be used in combination with setting CMAKE_INSTALL_PREFIX, but redirected to --prefix (essentially a copy operation), with the final package usable only if unpacked/copied to CMAKE_INSTALL_PREFIX.