docs: add comprehensive GitHub Copilot instructions for efficient agent development

[Copilot]

Adds a comprehensive .github/copilot-instructions.md file to help GitHub Copilot coding agents work efficiently with the xstate-audition repository without extensive exploration.

What This Adds

The instructions file provides coding agents with essential repository information including:

• Repository Overview: Description of xstate-audition as a dependency-free TypeScript library for testing XState v5+ Actors, with architecture details and target runtime requirements
• Complete Build & Validation Workflow: Step-by-step instructions for setup, building, testing, linting, and type checking with correct command sequences
• Project Layout: Detailed source structure, test organization, and configuration file locations
• Critical Issue Documentation: Fixes for known problems like the test script glob pattern issue in package.json

Key Problems Solved

Test Command Fix

The package.json test script uses "./test/*.spec.ts" which fails due to glob pattern handling. The instructions document the correct command:

node --test --import tsx test/*.spec.ts  # Works correctly

Build Dependencies

Documents that npm run build must always be run before linting since some lint rules check the file system for build outputs.

Complete Validation Sequence

Provides a tested sequence of commands that replaces the broken npm run check command:

npm run build
node --test --import tsx test/*.spec.ts
npm run test:types
npm run lint
npm run lint:knip
npm run lint:md
npm run lint:spelling

Validation

All instructions have been thoroughly tested:

• ✅ Build process works correctly with tshy
• ✅ Test commands run successfully with corrected glob pattern
• ✅ All linting steps pass (ESLint, knip, markdown, spelling)
• ✅ Type checking passes with TypeScript strict mode
• ✅ Instructions file itself passes all quality checks

The instructions follow the 2-page limit while providing comprehensive coverage of the repository's development workflow, significantly reducing the time agents need to explore and understand the codebase structure.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

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