feat: dagrunner, initial version

CONTRIBUTING.md

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+# Contributing to dagx
+
+Thanks for your interest in contributing! Please read our [Code of Conduct](CODE_OF_CONDUCT.md) before getting started.
+
+## Quick Start
+
+```bash
+# Clone and build
+git clone https://github.com/swaits/dagx
+cd dagx
+cargo build
+
+# Run tests
+cargo test
+
+# Check your code
+cargo fmt
+cargo clippy -- -D warnings
+```
+
+## Development Setup
+
+**Prerequisites:**
+- Rust 1.78+ (edition 2021)
+- Git
+
+**Optional tools:**
+```bash
+rustup component add rustfmt clippy
+cargo install cargo-llvm-cov  # For coverage
+cargo install cargo-audit     # For security checks
+```
+
+## Making Changes
+
+1. **Fork and branch**: Create a descriptive branch (`fix-xyz`, `feature-abc`)
+2. **Code**: Make your changes following our style guide
+3. **Test**: Add tests for new functionality
+4. **Document**: Update docs if needed
+5. **Verify**: Run the pre-flight checks below
+6. **PR**: Open a pull request with a clear description
+
+### Pre-flight Checklist
+
+Before submitting your PR, verify:
+
+```bash
+cargo test              # All tests pass
+cargo fmt -- --check    # Code is formatted
+cargo clippy -- -D warnings  # No clippy warnings
+cargo doc               # Docs build
+cargo build --examples  # Examples compile
+```
+
+## Code Style
+
+- **Format**: Use `rustfmt` (enforced by CI)
+- **Lint**: Fix all `clippy` warnings
+- **Document**: Add doc comments for public APIs
+- **Test**: Cover new code with tests
+
+### Documentation Guidelines
+
+- Document **why**, not **what**
+- Include examples for public APIs
+- Keep examples simple and focused
+- All doc examples must compile
+
+Example:
+```rust
+/// Adds a task to the DAG.
+///
+/// # Examples
+///
+/// ```
+/// use dagx::DagRunner;
+///
+/// let dag = DagRunner::new();
+/// let task = dag.add_task(MyTask::new());
+/// ```
+pub fn add_task<T: Task>(&self, task: T) -> TaskBuilder<T> {
+    // ...
+}
+```
+
+## Testing
+
+**Test organization:**
+- Unit tests: `#[cfg(test)]` modules in source files
+- Integration tests: `tests/` directory
+- Examples: `examples/` directory (must compile)
+- Doctests: In `///` comments (must pass)
+
+**Runtime testing:**
+```bash
+cargo test                           # Default (tokio)
+cargo test --test async_std_runtime  # async-std
+cargo test --test smol_runtime       # smol
+```
+
+**Benchmarks:**
+```bash
+cargo bench
+# View results: open target/criterion/report/index.html
+```
+
+## Continuous Integration
+
+Our CI runs on every PR and checks:
+
+- **Build**: Linux, macOS, Windows
+- **Test**: All tests across multiple runtimes
+- **Format**: `rustfmt` compliance
+- **Lint**: `clippy` with no warnings
+- **Coverage**: Minimum 80% code coverage
+- **Security**: `cargo audit` for vulnerabilities
+
+## Commit Guidelines
+
+Use clear, concise commit messages:
+
+```
+type: brief description
+
+Optional longer explanation if needed.
+
+Closes #123
+```
+
+**Types**: `feat`, `fix`, `docs`, `test`, `refactor`, `perf`, `chore`
+
+**Examples:**
+- `feat: add timeout support for tasks`
+- `fix: correct cycle detection in diamond DAG`
+- `docs: clarify TaskHandle usage in README`
+
+## Pull Requests
+
+**PR description should include:**
+- What changed and why
+- How it was tested
+- Any breaking changes
+- Link to related issue (if applicable)
+
+**Review process:**
+- Maintainer reviews within 1 week
+- Address feedback
+- Once approved, maintainer merges
+- Contributors are credited in releases
+
+## Getting Help
+
+- **Questions**: Open a GitHub Discussion
+- **Bugs**: Open a GitHub Issue
+- **Security**: Email steve@waits.net (see [SECURITY.md](SECURITY.md))
+
+## Release Process
+
+(For maintainers only)
+
+1. Update version in `Cargo.toml` and `CHANGELOG.md`
+2. Run full test suite: `cargo test --all-features`
+3. Tag release: `git tag -a v0.x.y -m "Release v0.x.y"`
+4. Push tag: `git push origin v0.x.y`
+5. Publish: `cargo publish`
+6. Create GitHub release with changelog
+
+---
+
+Thank you for contributing! 🎉