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.

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