Set up development environment (CPU-only surface + docs)

[lukass16]

Summary

Sets up the development environment for assembly-bench. This is a NIST-taskboard assembly benchmark built on Isaac Lab Arena / Isaac Sim 6.x, whose simulator requires an NVIDIA RTX GPU with no CPU fallback (docker/docker.md). The default Cursor Cloud VM has no NVIDIA GPU/driver/Vulkan, so the full simulator cannot run here — but the CPU-only, import-free benchmark surface (the manifest that hud eval/tasks.json consume) is installed and verified.

Changes

• Add AGENTS.md with a ## Cursor Cloud specific instructions section documenting the GPU/Isaac Sim hardware requirement, what works CPU-only, and the non-obvious submodule gotcha (submodules/IsaacLab-Arena is declared in .gitmodules but never committed as a gitlink, so git submodule update is a no-op).
• Ignore .venv/ in .gitignore.

Environment setup

• Update script (registered separately): creates .venv and pip install -e assembly_bench (idempotent, verified from a clean state).
• No source code changed.

Verified CPU-only (hello-world)

• from assembly_bench.environments.assembly.variants import VARIANTS → 27 variants across peg_insert (16), nut_thread (8), gear_mesh (3).
• Full scene/seat-geometry spec dump for the default task peg_round_8mm_tight.
• scripts/taskset.py regenerates tasks.json identically (idempotent).

Cannot run here (hardware limitation, not a bug)

• env.py, scripts/preview_assembly.py, Arena policy_runner.py/eval_runner.py, and the docker/Dockerfile env server all require an RTX GPU + Isaac Sim base image. See docker/docker.md to run on a GPU host.