docs: add CONTRIBUTING.md and simplify README contributing section

Add contributor guide covering prerequisites, setup, quality checks,
PR requirements, review standards reference, and project structure.
Simplify README contributing section to point to CONTRIBUTING.md.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: shinpr

CONTRIBUTING.md

@@ -0,0 +1,76 @@
+# Contributing to MCP Local RAG
+
+Contributions welcome! This guide covers what you need to get started.
+
+## Prerequisites
+
+- Node.js >= 20
+- [pnpm](https://pnpm.io/)
+
+## Setup
+
+```bash
+git clone https://github.com/shinpr/mcp-local-rag.git
+cd mcp-local-rag
+pnpm install
+```
+
+The embedding model (~90MB) downloads on first test/run.
+
+## Quality Checks
+
+All PRs must pass the full quality check, which mirrors CI:
+
+```bash
+pnpm run check:all
+```
+
+This runs the following in order:
+
+| Step | Command | What it checks |
+|------|---------|----------------|
+| Biome check | `pnpm run check` | Lint + format combined |
+| Lint | `pnpm run lint` | Code quality rules |
+| Format | `pnpm run format:check` | Code formatting |
+| Unused exports | `pnpm run check:unused` | No dead exports |
+| Circular deps | `pnpm run check:deps` | No circular dependencies |
+| Build | `pnpm run build` | TypeScript compilation |
+| Test | `pnpm run test` | All tests pass |
+
+Fix lint/format issues automatically:
+
+```bash
+pnpm run check:fix
+```
+
+## PR Requirements
+
+Before submitting a pull request:
+
+1. **Add tests** for new features and bug fixes
+2. **Run `pnpm run check:all`** and ensure everything passes
+3. **Update documentation** if behavior changes
+4. **Keep commits focused** — one logical change per PR
+
+## What We Look For
+
+This project's development standards — testing strategy, error handling, code organization, etc. — are documented in [claude-code-workflows/skills](https://github.com/shinpr/claude-code-workflows/tree/main/skills).
+
+We share this upfront so you know what to expect in review, not after. You don't need to memorize it, but if review feedback feels unexpected, that's where it comes from.
+
+## Project Structure
+
+```
+src/
+  index.ts        # Entry point
+  server/         # MCP tool handlers
+  parser/         # Document parsing (PDF, DOCX, TXT, Markdown, HTML)
+  chunker/        # Semantic text chunking
+  embedder/       # Transformers.js embeddings
+  vectordb/       # LanceDB operations
+  __tests__/      # Test suites
+```
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the [MIT License](LICENSE).

README.md

@@ -417,12 +417,7 @@ src/
 
 ## Contributing
 
-Contributions welcome. Before submitting a PR:
-
-1. Run tests: `pnpm test`
-2. Check quality: `pnpm run check:all`
-3. Add tests for new features
-4. Update docs if behavior changes
+Contributions welcome! See [CONTRIBUTING.md](CONTRIBUTING.md) for setup and guidelines.
 
 ## License