Add .github/copilot-instructions.md for Copilot coding agent onboarding

[Copilot]

Adds .github/copilot-instructions.md to give Copilot coding agents the context needed to work efficiently in this repo from first contact.

Contents

• Dev environment — Poetry setup, virtualenv, pre-commit installation
• Lint/format/test commands — exact black, flake8, pytest, and pre-commit invocations; Black config (79 chars, single quotes); flake8 ignored rules with explanations; 100% coverage requirement
• Code guidelines — docstrings-as-doctests, RST format, top-level imports, early returns, single-quote strings, no trailing whitespace
• Testing patterns — schema isolation workaround for Schema already has XXX errors; when to use doctests vs. tests/ unit tests
• Examples — required env vars (GH_TOKEN, SHOP_STORE, SHOP_TOKEN), NO_DOWNLOAD=1 workaround for schema regeneration without live tokens
• Common patterns — custom dunder methods (__str__, __add__, __iadd__, __fields__, __as__, etc.) used throughout the type system
• PR/commit conventions — atomic commits, pre-commit before every commit, human review required before opening PRs
• CI/CD summary and directory structure for quick orientation

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⌨️ Start Copilot coding agent tasks without leaving your editor — available in VS Code, Visual Studio, JetBrains IDEs and Eclipse.

[coveralls]

Pull Request Test Coverage Report for Build 23656473322

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 21763944764:	0.0%
Covered Lines:	1695
Relevant Lines:	1695

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💛 - Coveralls