Add .github/copilot-instructions.md for Copilot coding agent

[Copilot]

Set up Copilot coding agent instructions per best practices.

• Retained root AGENTS.md (verified all file references and claims against current codebase)
• Added .github/copilot-instructions.md covering project structure, build/test commands, F# coding conventions, API symmetry/wire-compatibility guardrails, Newtonsoft vs STJ behavioral differences, and testing practices

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[CLAassistant]

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[bartelink]

@copilot On reflection, please reinstate the AGENTS.md and check it's correct

[Copilot]

@copilot On reflection, please reinstate the AGENTS.md and check it's correct

Reinstated AGENTS.md in e3ed79e. Verified all file references and claims (SDK version, CI commands, file paths, coding patterns) against the current codebase — everything checks out.

[Copilot]

Pull request overview

Adds a dedicated Copilot Coding Agent instruction document for FsCodec to help automated agents/new contributors navigate repo structure, workflows, and key guardrails (API symmetry + wire-compat constraints).

Changes:

• Added .github/copilot-instructions.md with repo map, build/test commands, F# conventions, architecture guardrails, serializer differences, and testing notes.

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💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.