docs: add initial design document and CLAUDE.md

- DESIGN.md: Complete design specification for rnr task runner
- CLAUDE.md: Development guidelines and project structure

Author: CalvinAllen

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code when working with this repository.
+
+## Critical Rules
+
+1. **NEVER commit directly to main** - Always create a feature branch and submit a pull request
+2. **Conventional commits** - Format: `type(scope): description`
+3. **GitHub Issues for TODOs** - Use `gh` CLI to manage issues, no local TODO files
+4. **Pull Request titles** - Use conventional commit format
+5. **Branch naming** - Format: `type/scope/short-description` (e.g., `feat/parser/yaml-support`)
+6. **No co-authors** - Do not add co-author information on commits or pull requests
+7. **No "generated by" statements** - Do not add generated-by statements on pull requests
+
+## Project Overview
+
+**rnr** (pronounced "runner") is a cross-platform task runner designed for zero-setup execution. Clone a repo and tasks just work - no dependency installs, no global tools, no configuration.
+
+## Tech Stack
+
+- **Language**: Rust
+- **Target Platforms**: Windows, macOS, Linux
+- **Task File Format**: YAML (`rnr.yaml`)
+
+## Build Commands
+
+```bash
+# Build
+cargo build
+
+# Build release
+cargo build --release
+
+# Run tests
+cargo test
+
+# Run clippy
+cargo clippy
+
+# Format code
+cargo fmt
+
+# Run rnr locally
+cargo run -- <args>
+```
+
+## Project Structure
+
+```
+rnr.cli/
+├── src/
+│   ├── main.rs           # Entry point
+│   ├── cli.rs            # CLI argument parsing (clap)
+│   ├── config.rs         # rnr.yaml parsing (serde)
+│   ├── runner.rs         # Task execution
+│   ├── commands/
+│   │   ├── mod.rs
+│   │   ├── init.rs       # rnr init command
+│   │   ├── upgrade.rs    # rnr upgrade command
+│   │   └── list.rs       # rnr --list command
+│   └── ...
+├── Cargo.toml
+├── DESIGN.md             # Detailed design document
+├── CLAUDE.md             # This file
+└── README.md
+```
+
+## Architecture
+
+See `DESIGN.md` for complete design documentation including:
+- Task file schema
+- Built-in commands
+- MVP features
+- Future features
+
+## Conventions
+
+### Task File Schema
+
+Tasks are defined in `rnr.yaml`:
+
+```yaml
+# Shorthand
+build: cargo build --release
+
+# Full form
+test:
+  description: Run all tests
+  dir: src/
+  env:
+    RUST_LOG: debug
+  cmd: cargo test
+```
+
+### Error Handling
+
+- Use `anyhow` for application errors
+- Use `thiserror` for library errors
+- Provide clear, actionable error messages
+
+### Code Style
+
+- Follow Rust idioms
+- Use `clippy` for linting
+- Use `rustfmt` for formatting
+- Prefer explicit error handling over `.unwrap()`