✨ Set up AI agent instructions for the repository

[Copilot]

This PR adds comprehensive AI agent instructions to help AI coding agents and developers work more effectively with this repository.

What's Changed

Added AGENTS.md in the repository root with detailed guidance covering:

Repository Overview

• Explains this is a pure Haskell implementation of Google's Protocol Buffers
• Uses GHC.Generics for automatic encoding/decoding without requiring .proto files
• Supports Required, Optional, Repeated, and Packed fields with type-safe field tags

Build & Test Instructions

Clear commands for working with the Cabal build system:

cabal build --enable-tests --enable-benchmarks
cabal test --enable-tests
cabal check

Code Guidelines

• Language extensions commonly used (DeriveGeneric, DataKinds, OverloadedStrings, etc.)
• Naming conventions and code structure
• Documentation standards using Haddock

Project Architecture

• Core modules and their responsibilities
• Field types and key type classes (Encode, Decode, HasField)
• Testing approach using Tasty framework with QuickCheck and HUnit
• GHC version compatibility matrix (8.10.7 through 9.8.4)

Development Workflow

• Best practices before making changes
• Validation steps after changes
• Common pitfalls to avoid (wire format compatibility, field tag uniqueness, type-level programming gotchas)

Dependencies & Limitations

• Lists key dependencies and dependency management philosophy
• Documents known limitations (default values, extension fields, unknown fields)

This provides context-aware assistance for both AI agents and human contributors.

Fixes #43

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 #43

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[NathanHowell]

please rename this file to /AGENTS.md and remove any Copilot specific wording

[NathanHowell]

@copilot ptal

[Copilot]

Done. Renamed to /AGENTS.md and removed Copilot-specific wording. The title is now "AI Agent Instructions for protobuf" and the file is in the repository root. (33bcd15)