Add AGENTS.md with Cursor Cloud specific instructions

Author: cursoragent

AGENTS.md

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+# AGENTS.md
+
+## Cursor Cloud specific instructions
+
+### Overview
+
+NetGraph is a Python library + CLI tool for network modeling and analysis. It combines a Python front-end with C++ graph algorithms via `netgraph-core`. There are no web servers, databases, or Docker services.
+
+### Development commands
+
+All dev commands use the venv at `./venv`. See `Makefile` for the full list (`make help`).
+
+| Task | Command |
+|---|---|
+| Lint (ruff + pyright) | `make lint` |
+| Auto-format | `make format` |
+| Quick tests (no slow/benchmark) | `make qt` |
+| Full tests with coverage | `make test` |
+| Run a scenario | `ngraph run scenarios/square_mesh.yaml --output results/` |
+| Schema validation | `make validate` |
+
+### Gotchas
+
+- The system package `python3.12-venv` must be installed before `make dev` can create the virtualenv. The update script handles this.
+- `git config core.hooksPath` may be set by the Cloud Agent environment; if pre-commit hook installation fails with "Cowardly refusing to install hooks with `core.hooksPath` set", run `git config --unset-all core.hooksPath` first.
+- Pyright is pinned to `1.1.401` in `pyproject.toml`; ignore "new version available" warnings.
+- `make test` enforces a minimum 75% code coverage threshold (currently ~89%).