Add GitHub Copilot instructions

[Copilot]

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

Content

• Development setup: Bun requirements, bun dev, type checking
• Repository structure: Core packages and their purposes
• Agents: Build, plan, and general subagent capabilities
• Style conventions: Function composition, control flow, error handling, type precision, Bun APIs
• Contribution guidelines: PR expectations, accepted changes, feature request process

The instructions consolidate relevant content from CONTRIBUTING.md, STYLE_GUIDE.md, and AGENTS.md into a single reference for the coding agent.

Original prompt

---

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 #3

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