docs: add comprehensive migration guide for reusable workflows

## New Documentation

### MIGRATION.md (1000+ lines)

Complete migration guide with before/after examples for all workflow types:

**Docker Workflows (3 examples)**
- Simple Docker build (67% code reduction)
- Docker build with security scanning (71% code reduction)
- Multi-platform builds (70% code reduction)

**Security Workflows (3 examples)**
- Trivy vulnerability scanning (60% code reduction)
- Secret scanning with Gitleaks (56% code reduction)
- SBOM generation (61% code reduction)

**Kubernetes/Helm Workflows (3 examples)**
- Helm chart linting (60% code reduction)
- Helm chart testing (50% code reduction)
- Helm chart publishing (60% code reduction)

**Terraform Workflows (3 examples)**
- Terraform validation (56% code reduction)
- Terraform plan with Infracost (55% code reduction)
- Terraform apply with state backup (54% code reduction)

**GitOps Workflows (2 examples)**
- Complete GitOps pipeline (fully automated deployment)
- Helm values updates (automated PR creation)

**Migration Patterns**
- Parallel testing (run old + new side-by-side)
- Feature flag migration (test before enabling)
- Gradual rollout (migrate one job at a time)

**Troubleshooting Section**
- 6 common migration issues with solutions
- Repository not found errors
- Secret passing issues
- Permission errors
- Context availability
- Artifact handling
- Silent failures

**Migration Checklist**
- Pre-migration checklist
- During migration checklist
- Post-migration checklist
- Rollback plan

### README.md Update

- Removed "coming soon" note from Migration Guide
- Updated description to reflect all workflow types

## Benefits for Users

**Quantified Code Reduction:**
- Average 60% reduction in workflow YAML code
- Single source of truth for CI/CD patterns
- Automatic action updates via Renovate
- Consistent security practices

**Migration Support:**
- Clear before/after examples
- Step-by-step migration strategy
- Risk mitigation patterns
- Comprehensive troubleshooting

**Documentation Complete:**
- ✅ README.md (quick start)
- ✅ USAGE.md (parameter documentation)
- ✅ EXAMPLES.md (complete workflow examples)
- ✅ MIGRATION.md (before/after migration guide)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: samuelho-dev

README.md

@@ -132,7 +132,7 @@ jobs:
 
 - **[Usage Guide](docs/USAGE.md)** - Detailed usage instructions for all workflows
 - **[Examples](docs/EXAMPLES.md)** - Complete workflow examples and patterns
-- **[Migration Guide](docs/MIGRATION.md)** - Migrate from inline Docker commands (coming soon)
+- **[Migration Guide](docs/MIGRATION.md)** - Migrate from inline commands to reusable workflows
 
 ## 🔧 Configuration