feat: add spec-driven development example using markdown agents

[jellydn]

What

Add a comprehensive spec-driven development example folder that demonstrates mdflow's full capabilities. Includes seven interconnected markdown agents for structured feature development:

• constitution.claude.md - Establish project principles with dynamic variables and file imports
• specify.claude.md - Interactive requirements workshop with file references
• clarify.claude.md - Clarification workshop for underspecified areas
• plan.claude.md - Technical architecture with inline commands and project analysis
• tasks.claude.md - Actionable task breakdown with current state analysis
• review.claude.md - Quality gates and consistency checks across all specs
• implement.claude.md - Interactive task execution guidance with validation

Plus comprehensive README explaining patterns and customization.

Why

This demonstrates that mdflow is more than a CLI tool—it's a framework for structured AI-driven development workflows. Shows:

1. mdflow features in action: Variables, file imports, inline commands, piping, interactive mode
2. Spec-driven development: Lightweight alternative to GitHub's Spec Kit without extra frameworks
3. Agent composition: How to chain agents together and cross-reference specifications
4. Real-world patterns: Customization examples, team workflows, quality gates
5. Template reusability: Copy-paste templates for new projects

Makes mdflow immediately useful for teams adopting structured development practices.

How

Key Features Demonstrated

Template Variables
```yaml
_project: "{{ _project | default: 'MyProject' }}"
_team: "{{ _team | default: 'Engineering Team' }}"
```

File Imports
```markdown
@./package.json
@./src/**/*.ts
@./tsconfig.json:1-20
```

Inline Commands
```markdown
!npm test
!git log --oneline -5
!npm list --depth=0
```

Interactive Mode & Cross-References

See README.md for complete examples, customization guide, and comparison to Spec Kit.

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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