Add Copilot setup workflow and agent documentation

[Copilot]

Add copilot setup yml and lean AGENTS.md inspired by cross-org/kv

Plan:

• Create .github/workflows/copilot-setup-steps.yml based on kv repo
  • Adapt to test repo's specific needs (Deno, Node.js, Bun)
  • Follow same structure with checkout, setup steps, and dependency caching
• Create AGENTS.md in repository root as lean documentation
  • Adapt kv's agents checklist to test repo specifics
  • Include CI reference, pre-commit checks, and guidelines
  • Keep it concise and focused on cross-runtime testing framework
• Verify files are properly created and formatted
• Address code review feedback
  • Fixed step name from "Cache dependencies" to "Install dependencies"
  • Removed misleading package.json reference (repo uses deno.json)
• Fix formatting issues
  • Fixed AGENTS.md to pass Deno fmt checks
  • Joined multi-line text entries per Deno formatting rules

Completed:

• Created .github/workflows/copilot-setup-steps.yml with Deno, Node.js, and Bun setup
• Created AGENTS.md with lean checklist adapted for test repository
• Both files follow the same structure and patterns from cross-org/kv
• Addressed all code review comments
• Fixed Deno formatting issues in AGENTS.md

Original prompt

Add copilot setup yml and a lean agents.md inspired by the implementations in cross-org/kv

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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.

[Hexagon]

@copilot fix formatting, ci is failing

[Copilot]

@copilot fix formatting, ci is failing

Fixed formatting in AGENTS.md to pass Deno fmt checks. The issue was multi-line text that needed to be joined per Deno formatting rules (850f399).