feat(expansion): Initialize TDD methodology expansion pack structure

- Created expansion pack directory structure
- Added README with comprehensive documentation
- Prepared directories for agents, tasks, templates, scripts, and examples

This expansion pack adds TDD capabilities to the BMAD-METHOD, including:
- Enhanced QA and Dev agents for TDD phases
- TDD-specific commands and workflows
- Quality gates and CI/CD integration
- Example implementations

expansion-packs/tdd-methodology/README.md

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+# BMAD-METHOD™ TDD Methodology Expansion Pack
+
+This expansion pack enhances the BMAD-METHOD™ with comprehensive Test-Driven Development (TDD) capabilities, enabling teams to follow strict TDD practices with AI assistance.
+
+## Features
+
+- Enhanced QA and Dev agent personas with TDD-specific responsibilities
+- TDD-aware test design tasks and templates
+- Full Red-Green-Refactor cycle support
+- TDD quality gates and validation
+- CI/CD integration for TDD enforcement
+- Practical examples and demos
+
+## Components
+
+### Agent Enhancements
+
+- QA Agent: Enhanced for TDD Red phase and test creation
+- Dev Agent: Enhanced for TDD Green phase implementation
+
+### New Commands
+
+- `*tdd-start`: Initialize TDD workflow for a story
+- `*write-failing-tests`: Generate failing tests (Red phase)
+- `*tdd-implement`: Implement code to make tests pass (Green phase)
+- `*tdd-refactor`: Safe refactoring with test coverage
+
+### Quality Gates
+
+- Phase-specific quality criteria
+- Automated validation through CI/CD
+- TDD discipline enforcement
+
+### Templates and Tasks
+
+- Enhanced test design task with TDD support
+- TDD quality gates template
+- CI/CD workflow templates
+
+### Guard Scripts
+
+- TDD discipline validation
+- Git diff inspection
+- CI pipeline integration
+
+## Installation
+
+1. Copy the contents of this expansion pack to your BMAD-METHOD implementation
+2. Configure your CI/CD pipeline using the provided templates
+3. Update your agent configurations to include TDD capabilities
+
+## Usage
+
+1. Initialize TDD mode for a story:
+
+   ```
+   *tdd-start "Story description"
+   ```
+
+2. Follow the Red-Green-Refactor cycle:
+   - Red: `*write-failing-tests`
+   - Green: `*tdd-implement`
+   - Refactor: `*tdd-refactor`
+
+3. Monitor quality gates and CI/CD pipeline for TDD compliance
+
+## Example
+
+See the `examples/tdd-demo` directory for a complete demonstration of the TDD workflow using the "User Email Validation" story.
+
+## Configuration
+
+The TDD methodology can be enabled/disabled per project or story through configuration flags. See the documentation for detailed configuration options.
+
+## Contributing
+
+Contributions to improve the TDD methodology expansion pack are welcome. Please follow the standard BMAD-METHOD contribution guidelines.
+
+## License
+
+This expansion pack is released under the same license as the BMAD-METHOD™.