LLM based coding documentation

[greole]

This PR adds a first AGENTS.md and instructions for LLM based coding. I think we can gradually extend the AGENTS.md file.

Additionally, to avoid flooding the repo with tool specific stuff CLAUDE.md and .planning are added to gitignore.

[github-actions]

Deployed test documentation to https://exasim-project.com/NeoFOAM/Build_PR_279

[HendriceH]

LGTM!

[chihtaw]

LGTM!
The only major thing I think is missing is explicit GPU execution guidance.

[chihtaw]

Consider adding a section about GPU execution semantics/Kokkos constraints

Since NeoFOAM heavily relies on NeoN/Kokkos portability, we can add something like

## GPU considerations
- Device kernels must use NEON_LAMBDA
- Avoid host-only allocations/access insider kernels
- Prefer parallelFor abstractions over raw loops
- Explicit synchronization may be required before host reads ( NeoN::fence(exec) )
- Use .copyToHost() when validating vectors in tests

[greole]

Good point so far, no one is using Claude or anything else on a GPU node, but this will come at some point.

[chihtaw]

Consider documenting testing conventions more explicitly

Suggested change

	Each test file starts an OpenFOAM `Time`/mesh in `test_main.cpp`, then Catch2 test cases use `GENERATE(allAvailableExecutor())` to run the same case on every compiled executor (Serial, CPUExecutor for OpenMP/threads, GPUExecutor for CUDA/HIP/SYCL). Setup case files live in `test/setup_<name>/`.
	Tests should generally validate behavior across all enabled executors (CPU/GPU) and avoid assumptions about deterministic floating-point ordering.
	Each test file starts an OpenFOAM `Time`/mesh in `test_main.cpp`, then Catch2 test cases use `GENERATE(allAvailableExecutor())` to run the same case on every compiled executor (Serial, CPUExecutor for OpenMP/threads, GPUExecutor for CUDA/HIP/SYCL). Setup case files live in `test/setup_<name>/`.

[chihtaw]

The current wording might unintentionally imply a Claude-specific workflow. It could be slightly broader.

Suggested change

	Please instruct tools such as Claude Code to reference this file, for example by prompting: “See `AGENTS.md` for shared project instructions.”
	Please instruct AI coding tools to reference this file, for example by prompting: “See `AGENTS.md` for shared project instructions.”