Add Claude Code config

Author: robtaylor

CLAUDE.md

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+# ChipFlow Library Development Guidelines
+
+## Build & Test Commands
+- Install dependencies: `pdm install`
+- Run all tests: `pdm run test`
+- Run a single test: `pdm run python -m pytest tests/test_file.py::test_function -v`
+- Run tests with coverage: `pdm run test-cov`
+- Run linting: `pdm run lint`
+- Build documentation: `pdm run docs`
+- Test documentation: `pdm run test-docs`
+
+## Code Style Guidelines
+- License header: Include `# SPDX-License-Identifier: BSD-2-Clause` at the top of each file
+- Use type hints where appropriate (Python 3.10+ supported)
+- Imports: Group standard library, third-party, and project imports with a blank line between groups
+- Formatting: Project uses ruff (`F403`, `F405` are ignored)
+- Naming: Use snake_case for functions/variables and PascalCase for classes
+- Error handling: Custom errors inherit from `ChipFlowError`
+- Documentation: Follow sphinx docstring format for public APIs
+- Testing: Use pytest fixtures and assertions