Add architectural guidance for configuration system in CONTRIBUTING.md

## Problem Several open feature requests exist around the configuration system (#5849, #5782, #5765), but contributors lack documented guidance on how the configuration architecture works and where to implement changes. This creates friction for implementing features like: - STRYKER_CONFIG_PATH environment variable support - Making MAX_CONCURRENT_INIT configurable - Adding new mutators Without this guidance, contributors must reverse-engineer the config system by reading source code, leading to slower, less consistent implementations. ## What's Missing The CONTRIBUTING.md file should document: 1. **Configuration Flow**: How does a config option flow from user input (CLI args, env vars, config file) → parsed schema → runtime behavior? 2. **Where Configuration Lives**: Map of key files: - `packages/core/src/config/` structure and responsibilities - Which files handle parsing vs validation vs defaults - Where environment variables are read (if at all currently) 3. **How to Add a Config Option**: Step-by-step walkthrough: - Update the schema (what file?) - Add validation logic - Wire it to the runtime code - Add tests 4. **Mutator Registration**: How do mutators register themselves with the core system? ## Why This Matters - **Contributor Onboarding**: New contributors can self-serve instead of asking maintainers - **Consistency**: Ensures all config options follow the same pattern - **Quality**: Reduces chances of incomplete implementations missing validation or tests - **Speed**: PRs get merged faster when the approach is pre-validated ## What Good Looks Like Look at how other projects document this (e.g., TypeScript's architecture overview, ESLint's plugin development guide). A minimal version would include: ``` ## Architecture: Configuration System ### Overview [1-2 paragraphs explaining the flow] ### Key Files - `packages/core/src/config/schema.ts` — JSON schema definition - `packages/core/src/config/configReader.ts` — Loads and parses config [...] ### Adding a Config Option 1. Update schema in `schema.ts` 2. Add validation in `configValidator.ts` 3. Wire to usage in `src/runner.ts` 4. Add test in `test/unit/config/` ### Example: Adding `maxConcurrentInit` [Walkthrough of actual PR or commit that added a recent config option] ``` ## Acceptance Criteria - [ ] CONTRIBUTING.md has a dedicated "Configuration System" section - [ ] Section includes config flow diagram (ASCII art OK) - [ ] Key files mapped with 1-line descriptions of responsibility - [ ] Step-by-step "Adding a Config Option" guide with real example - [ ] Links to relevant source files or recent related PRs --- *Contributed by [Klement Gunndu](https://github.com/KlementMultiverse)*