Add grader showcase examples demonstrating all grader types (microsoft#134)

Adds grader showcase examples with all 5 grader types, fixtures, and comprehensive README. Closes microsoft#108.

Author: Copilot

examples/README.md

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+# Waza Examples
+
+This directory contains example evaluation suites demonstrating various features and use cases of waza.
+
+## Available Examples
+
+### 1. [code-explainer](./code-explainer/)
+**Purpose**: Complete eval suite for a skill that explains code snippets
+
+**Demonstrates**:
+- Multi-trial testing across different programming languages
+- Custom script graders for complex evaluation logic
+- Task organization with YAML files
+- Metrics definition and weighting
+
+**Quick Start**:
+```bash
+waza run examples/code-explainer/eval.yaml --context-dir examples/code-explainer/fixtures -v
+```
+
+---
+
+### 2. [grader-showcase](./grader-showcase/)
+**Purpose**: Comprehensive demonstration of all grader types
+
+**Demonstrates**:
+- `code` grader: Python assertion-based validation
+- `regex` grader: Pattern matching in output
+- `file` grader: File existence and content validation
+- `behavior` grader: Agent behavior constraints
+- `action_sequence` grader: Tool call sequence validation
+
+**Quick Start**:
+```bash
+waza run examples/grader-showcase/eval.yaml --context-dir examples/grader-showcase/fixtures -v
+```
+
+**What's Inside**:
+- 5 task examples, one for each grader type
+- Detailed README with configuration options
+- Runnable fixtures and test data
+- Examples of global vs. task-specific graders
+
+---
+
+### 3. [ci](./ci/)
+**Purpose**: GitHub Actions workflow examples for CI/CD integration
+
+**Demonstrates**:
+- Basic waza evaluation in CI pipeline
+- Matrix testing across multiple models
+- Result comparison and reporting
+- Reusable workflow patterns
+
+**Quick Start**:
+See [ci/README.md](./ci/README.md) for integration instructions.
+
+---
+
+## Usage Patterns
+
+### Running Examples
+
+Run an entire eval suite:
+```bash
+waza run examples/<example-name>/eval.yaml -v
+```
+
+Run with context directory (for file-based tasks):
+```bash
+waza run examples/<example-name>/eval.yaml --context-dir examples/<example-name>/fixtures -v
+```
+
+Run specific tasks only:
+```bash
+waza run examples/<example-name>/eval.yaml --filter="task-name-pattern" -v
+```
+
+Save results to JSON:
+```bash
+waza run examples/<example-name>/eval.yaml -o results.json
+```
+
+### Using Examples as Templates
+
+1. **Copy an example**:
+   ```bash
+   cp -r examples/grader-showcase my-eval
+   cd my-eval
+   ```
+
+2. **Modify for your use case**:
+   - Update `eval.yaml` with your skill name and config
+   - Edit or create task files in `tasks/`
+   - Add fixture files to `fixtures/`
+   - Adjust graders to match your validation needs
+
+3. **Run your eval**:
+   ```bash
+   waza run eval.yaml -v
+   ```
+
+## Example Comparison
+
+| Example | Best For | Complexity | Grader Types |
+|---------|----------|------------|--------------|
+| **code-explainer** | Complete real-world eval | Medium | code, regex, script |
+| **grader-showcase** | Learning grader types | Low | All types |
+| **ci** | GitHub Actions integration | Low | N/A (workflow examples) |
+
+## Learning Path
+
+1. **Start with grader-showcase** to understand grader types
+2. **Study code-explainer** for realistic eval structure
+3. **Reference ci** for production integration
+
+## File Structure Convention
+
+Most examples follow this structure:
+```
+example-name/
+├── eval.yaml           # Main benchmark spec
+├── README.md           # Documentation
+├── fixtures/           # Context files for tasks
+│   ├── file1.ext
+│   └── file2.ext
+├── tasks/              # Individual task definitions
+│   ├── task-1.yaml
+│   └── task-2.yaml
+└── graders/            # Optional: custom script graders
+    └── custom_grader.py
+```
+
+## Related Documentation
+
+- **Grader Reference**: [docs/GRADERS.md](../docs/GRADERS.md)
+- **Main README**: [README.md](../README.md)
+- **Implementation Details**: [IMPLEMENTATION.md](../IMPLEMENTATION.md)
+
+## Contributing Examples
+
+To add a new example:
+
+1. Create a directory: `examples/your-example/`
+2. Add required files: `eval.yaml`, `README.md`, `tasks/`, `fixtures/`
+3. Document in this README
+4. Test with `waza run examples/your-example/eval.yaml -v`
+5. Submit a PR
+
+## Support
+
+- **Issues**: Open an issue on GitHub
+- **Discussions**: Check existing examples for patterns
+- **Documentation**: See linked docs above