Add comprehensive Copilot coding agent instructions

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file to onboard the repository for use with GitHub Copilot coding agents. The instructions are designed to help coding agents work more efficiently by providing essential context about the codebase, build process, and architecture.

What's included

Repository Overview: Describes slimcontext as a lightweight, model-agnostic chat history compression library using a "Bring Your Own Model" (BYOM) approach. Documents that it's a TypeScript npm library with ~24 source files targeting Node.js CommonJS modules.

Build Instructions: Provides a complete development workflow with validated commands:

• Dependency installation with npm (fallback when pnpm unavailable)
• Build process using TypeScript compiler
• Testing with vitest (7 tests across 2 files)
• Code formatting with Prettier
• Known ESLint configuration issue documented with workaround

Project Architecture: Details the framework-agnostic design with core interfaces (SlimContextMessage, SlimContextChatModel, SlimContextCompressor) and two main compression strategies (trim and summarize). Maps out the directory structure and explains the role of key files.

Validation Steps: Documents expected test outputs, build artifacts, and common issues with their workarounds.

Verification

All documented commands have been tested and verified to work correctly:

• npm install → successful dependency installation with automatic build
• npm run test → 7 tests pass across 2 files as documented
• npm run format:check → code style validation passes
• npm run build → generates expected dist/ artifacts
• npm run lint → confirms documented ESLint issue exists but doesn't affect functionality

The instructions follow the 2-page limit and focus on reducing agent exploration time by providing comprehensive context about build processes, architecture, and known issues upfront.

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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.

[IBJunior]

Everything is good for this first version of .github/copilot-instructions.md, it'll probably evolve with new versions.

[Copilot]

Pull Request Overview

This PR adds comprehensive GitHub Copilot instructions to help coding agents work more efficiently with the slimcontext repository by providing essential context about the codebase, build process, and architecture.

• Documents the repository as a TypeScript npm library for chat history compression with a "Bring Your Own Model" approach
• Provides validated development workflow with dependency installation, build process, testing, and formatting commands
• Details project architecture including core interfaces and compression strategies with known ESLint configuration issue workaround

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_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}