Add comprehensive Copilot coding agent instructions

[Copilot]

This PR adds .github/copilot-instructions.md to onboard the repository for Copilot coding agents. The instructions provide comprehensive guidance to help agents work efficiently without extensive exploration.

What's Included

Project Overview: Clear description of the Mode Python SDK, its purpose (trading API client), architecture, and key statistics (8 Python files, modern tooling with uv/pydantic/ruff).

Build & Development Workflow: Step-by-step instructions for:

• Environment setup with uv venv and virtual environment activation
• Dependency installation with proper extras ([dev], [backtest], [dev,backtest])
• Validation pipeline with pytest, black, ruff, and mypy
• Common issues and their solutions

Project Architecture: Detailed breakdown of:

• Directory structure and key components
• Main entry points (ModeAPIClient, resources, models)
• Authentication flow and configuration
• Pydantic model validation patterns

CI/CD Pipeline: Documentation of GitHub workflows:

• tests.yml - Runs pytest with multiple dependency combinations
• quality.yml - Runs code formatting, linting, and type checking
• Validation requirements for pull requests

Quick Reference: Command cheat sheet for immediate productivity.

Validation

All instructions have been thoroughly tested in a clean environment to ensure:

• ✅ All setup commands work correctly
• ✅ All validation commands pass (16 tests, formatting, linting, type checking)
• ✅ Both [dev] and [dev,backtest] installation combinations work
• ✅ Instructions are accurate and up-to-date

This will significantly reduce the time agents spend exploring the codebase and prevent common build/validation failures, leading to higher quality pull requests.

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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.