Set up Copilot instructions

[Copilot]

Adds .github/copilot-instructions.md to configure Copilot coding agent behavior for this repository.

Instructions include:

• Project structure overview (core natura lib, bevy-natura plugin, examples)
• Build and test commands
• Rust code style conventions and physics-related naming patterns
• Architecture overview of key types (Spring, DeltaTime, AngularFrequency, etc.)
• Guidelines for making changes

Original prompt

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This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #4

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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.