Add AGENTS.md with Cursor Cloud development instructions

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Adds AGENTS.md with development environment instructions for Cursor Cloud agents.

What's included

• Instructions for running all three scripts (train.py, neuro_symbolic_vector_graph_prototype.py, view_kb_graph.py)
• Non-obvious caveats: HuggingFace model download on first run, matplotlib backend handling, tqdm in non-TTY environments
• No code changes to the existing codebase

Evidence of working environment

Graph Transformer training (train.py --steps 500):
train_output.log

Neuro-symbolic prototype (1 epoch, small subset):
proto_output.log

KB graph visualization:
Knowledge base graph visualization

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