Add comprehensive GitHub Copilot instructions with development best practices for ReactPy

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot with detailed instructions for working effectively in the ReactPy codebase, incorporating best practices from established projects.

What's Included

Enhanced Development Guidance:

• Bug investigation workflow prioritizing docs/source/about/changelog.rst before Git history
• Clear testing standards with "All tests must always pass" expectations
• Interactive development shell instructions using hatch shell
• Structured 10-step development workflow with documentation and changelog requirements

Comprehensive Command Reference:

hatch test                          # Run all tests (must always pass)
hatch test --cover                  # Run tests with coverage (CI)
hatch run javascript:build          # Build JavaScript packages (15s)
hatch run python:type_check         # Type checking (10s)
hatch fmt --check                   # Format validation (1s)

Detailed Repository Navigation:

• Enhanced directory structure with purpose explanations
• Key file identification and dependency breakdown
• Package architecture covering Python-to-JavaScript bridge components

Safety and Standards:

• Explicit "NEVER CANCEL" warnings with 60+ minute timeout recommendations
• Build timing expectations for all development commands
• Validation requirements for documentation updates and changelog entries

Validation Scenarios: Working code examples for common development tasks including component creation, ASGI server deployment, React hooks, and FastAPI integration.

Key Improvements Over Initial Version

• Bug Investigation: Clear prioritization of changelog over Git history
• Testing Standards: Explicit expectation that all tests must pass
• Development Workflow: Numbered steps including documentation requirements
• Command Reference: Dedicated section for daily development commands
• Repository Structure: Enhanced navigation with detailed explanations
• Package Dependencies: Clear breakdown of runtime vs optional dependencies
• CI/CD Details: Comprehensive pipeline description with build matrix info

All commands and examples have been validated to ensure reliability and accuracy.

Fixes #1302.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

@Archmonger 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.

[Archmonger]

@copilot Take inspiration from django-dbbackup's instruction file to add additional information.

[Copilot]

@copilot Take inspiration from django-dbbackup's instruction file to add additional information.

I've enhanced the GitHub Copilot instructions by incorporating best practices from django-dbbackup. Added sections for bug investigation (prioritizing changelog.rst), testing standards, interactive development shell, structured development workflow, detailed repository navigation, common commands reference, package dependencies breakdown, and important notes. The instructions now provide comprehensive guidance with validated commands and clear expectations. See commit 8ea5868.