docs: add agent guidance

AGENTS.md

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+# AGENTS.md
+
+## Repo purpose
+
+`qref` defines the Quantum Resource Estimation Format and provides validation, schema tooling, docs, and rendering utilities.
+
+## Setup prerequisites
+
+- Python 3.10-3.12
+- `poetry`
+- Graphviz on `PATH` for rendering-related workflows
+
+## Canonical local commands
+
+- Install default environment: `poetry install`
+- Install docs environment: `poetry install --with docs`
+- Lint/hooks: `pre-commit run -a`
+- Unit tests: `poetry run pytest --json-report --json-report-file=pytest_report.json --json-report-omit collectors log streams --cov src --cov-report term --cov-report html`
+- Type checks: `MYPYPATH=src poetry run mypy src --install-types --non-interactive`
+- Docs build: `poetry run mkdocs build`
+
+## Validation before handoff
+
+- Run `pre-commit run -a` for normal code changes.
+- Run the pytest command for library or schema changes.
+- Run the mypy command when changing typed library code.
+- Run `poetry run mkdocs build` when touching docs or `mkdocs.yml`.
+
+## Risky or restricted areas
+
+- Keep GitHub Actions and release automation aligned with the existing GitHub workflow model.
+- Schema or validation changes can affect downstream consumers broadly.
+- Graphviz-dependent rendering paths may fail without the system binary.
+
+## Repo-specific notes for agents
+
+- This repo uses GitHub Actions and an open-source-style workflow rather than the internal GitLab conventions used by most other repos here.
+- Pre-commit uses `isort`, `black`, and `flake8`, not Ruff.
+- Preserve compatibility expectations for the QREF format unless the task explicitly changes schema behavior.