Add comprehensive Copilot coding agent instructions

.github/copilot-instructions.md

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+# Copilot Instructions for slimcontext
+
+## Repository Overview
+
+**slimcontext** is a lightweight, model-agnostic chat history compression library for AI assistants. It provides simple strategies to keep conversations concise while preserving context using a "Bring Your Own Model" (BYOM) approach.
+
+### High-Level Repository Information
+
+- **Type**: TypeScript npm library/package
+- **Source Files**: ~24
+- **Installed Size**: ~106MB (including all dependencies in node_modules)
+- **Languages**: TypeScript (primary), JavaScript (compiled output)
+- **Target Runtime**: Node.js (CommonJS modules)
+- **Framework**: Model-agnostic, no AI framework dependencies
+- **Package Manager**: pnpm (preferred) or npm (fallback)
+- **Testing**: vitest
+- **Linting**: ESLint with TypeScript support
+- **Formatting**: Prettier
+
+## Build Instructions and Development Workflow
+
+### Prerequisites and Environment Setup
+
+- **Node.js**: Version 20+ (as specified in the `engines` field of package.json)
+- **Package Manager**: pnpm 10.14.0 (preferred) or npm (fallback)
+
+### Critical Build Steps (Always Follow This Order)
+
+1. **Install Dependencies**
+
+   ```bash
+   # Preferred method (if pnpm available):
+   pnpm install --frozen-lockfile
+
+   # Fallback method (always works):
+   npm install
+   ```
+
+   **Always run install before any other commands.** The `prepare` script automatically runs build after install.
+
+2. **Build the Project**
+
+   ```bash
+   npm run build
+   # Compiles TypeScript to dist/ directory
+   # Duration: ~5-10 seconds
+   ```
+
+3. **Run Tests**
+
+   ```bash
+   npm run test
+   # Runs all vitest tests
+   # Duration: ~5-10 seconds
+   # Should show: "Test Files 2 passed (2), Tests 7 passed (7)"
+   ```
+
+4. **Format Code**
+
+   ```bash
+   npm run format        # Auto-format code
+   npm run format:check  # Check formatting without changes
+   ```
+
+5. **Lint Code (Known Issue)**
+   ```bash
+   npm run lint
+   ```
+   **KNOWN ISSUE**: ESLint currently fails with "parserOptions.tsconfigRootDir must be an absolute path" error. This is a configuration bug but doesn't affect build or tests. The code itself is properly linted in CI environment.
+
+### Complete Development Workflow
+
+```bash
+# Clean start (recommended for agents):
+rm -rf node_modules dist
+npm install           # Always use npm for reliability
+npm run test         # Verify tests pass
+npm run format:check # Verify formatting
+npm run build        # Final build
+```
+
+### CI/CD Pipeline Validation
+
+The repository uses GitHub Actions CI that runs:
+
+1. `pnpm install --frozen-lockfile`
+2. `pnpm run lint`
+3. `pnpm run format:check`
+4. `pnpm run build`
+5. `pnpm test`
+
+**Note**: CI works because it runs in a different environment where the ESLint path issue doesn't occur.
+
+## Project Layout and Architecture
+
+### Core Directory Structure
+
+```
+/
+├── src/                      # TypeScript source code
+│   ├── index.ts             # Main exports (TrimCompressor, SummarizeCompressor, interfaces)
+│   ├── interfaces.ts        # Core type definitions (SlimContextMessage, etc.)
+│   └── strategies/          # Compression strategy implementations
+│       ├── trim.ts          # TrimCompressor: keeps first + last N messages
+│       └── summarize.ts     # SummarizeCompressor: AI-powered summarization
+├── tests/                   # vitest test files
+│   ├── trim.test.ts        # Tests for TrimCompressor
+│   └── summarize.test.ts   # Tests for SummarizeCompressor
+├── examples/               # Documentation-only examples (not code)
+│   ├── OPENAI_EXAMPLE.md  # Copy-paste OpenAI integration
+│   └── LANGCHAIN_EXAMPLE.md # Copy-paste LangChain integration
+├── dist/                   # Compiled JavaScript output (generated)
+└── package.json           # npm package configuration
+```
+
+### Configuration Files
+
+- **tsconfig.json**: TypeScript compiler configuration (target: ES2019, CommonJS)
+- **tsconfig.eslint.json**: Extended config for ESLint (includes tests)
+- **.eslintrc.json**: ESLint configuration with TypeScript, import rules
+- **.prettierrc.json**: Code formatting rules (single quotes, trailing commas)
+- **.github/workflows/ci.yml**: CI/CD pipeline definition
+
+### Key Architecture Elements
+
+**Core Interfaces** (src/interfaces.ts):
+
+- `SlimContextMessage`: Standard message format with role ('system'|'user'|'assistant'|'tool'|'human') and content
+- `SlimContextChatModel`: BYOM interface requiring only `invoke(messages) -> response`
+- `SlimContextCompressor`: Strategy interface for compression implementations
+
+**Compression Strategies**:
+
+- **TrimCompressor**: Simple strategy keeping first (system) message + last N-1 messages
+- **SummarizeCompressor**: AI-powered strategy that summarizes middle conversations when exceeding maxMessages
+
+**Framework Independence**: No dependencies on OpenAI, LangChain, or other AI frameworks. Users implement the minimal `SlimContextChatModel` interface to connect their preferred model.
+
+### Dependencies and Build Artifacts
+
+- **Production**: Zero dependencies (framework-agnostic design)
+- **Development**: TypeScript, ESLint, Prettier, vitest, various type definitions
+- **Ignored Files**: dist/, node_modules/, examples/ (linting), \*.tgz
+- **Distributed Files**: Only dist/ directory (compiled JS + .d.ts files)
+
+## Validation and Testing
+
+### Running Tests
+
+```bash
+npm run test
+# Expects: 7 tests across 2 files, all passing
+# Tests cover both TrimCompressor and SummarizeCompressor functionality
+```
+
+### Manual Verification Steps
+
+1. Check that dist/ contains compiled .js and .d.ts files after build
+2. Verify examples/ directory contains markdown documentation (not executable code)
+3. Ensure src/ exports work by checking dist/index.js contains proper CommonJS exports
+
+### Common Issues and Workarounds
+
+1. **ESLint Path Error**: Use npm instead of pnpm locally, or ignore lint failures for development (CI handles this correctly)
+2. **Missing pnpm**: Always fall back to npm install - works identically for this project
+3. **Build Failures**: Always run `npm install` first - the prepare script ensures build runs after dependency installation
+
+## Key Files Reference
+
+### Root Directory Files
+
+- **README.md**: Comprehensive usage documentation with examples
+- **package.json**: Defines scripts, dependencies, and npm package metadata
+- **CHANGELOG.md**: Version history and breaking changes documentation
+- **LICENSE**: MIT license
+- **pnpm-lock.yaml**: pnpm lockfile (prefer npm for compatibility)
+
+### Source Code Entry Points
+
+- **src/index.ts**: Main library exports - start here for code changes
+- **src/interfaces.ts**: Core type definitions - modify for interface changes
+- **src/strategies/trim.ts**: Simple compression logic
+- **src/strategies/summarize.ts**: AI-powered compression with alignment logic
+
+---
+
+**Trust these instructions first** - only search for additional information if these instructions are incomplete or found to be incorrect. The documented commands and workflows have been validated in the development environment.