Add CI/CD pipeline configuration skill (azd pipeline config, GitHub Actions, Azure DevOps)

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## Summary

There is no skill covering CI/CD pipeline setup for Azure deployments. This is a gap identified from an analysis of proposed azd skills (Azure/azure-dev#6501).

## Proposed Skill: pipeline-config

**Trigger scenarios:**
- Setting up CI/CD with \zd pipeline config\
- Generating GitHub Actions workflows for Azure deployments
- Configuring Azure DevOps pipelines
- Setting up service principals, federated credentials, and secrets
- Managing deployment environments and approvals

**Coverage needed:**
- \zd pipeline config\ workflow (provider selection, credential setup)
- GitHub Actions workflow generation (build, test, deploy stages)
- Azure DevOps pipeline YAML generation
- Service principal / federated identity credential guidance
- Secrets management (GitHub Secrets, Azure Key Vault integration)
- Multi-environment deployment patterns (dev/staging/prod)
- Deployment gates and approval flows

**Reference:** The \zure-prepare\ and \zure-deploy\ skills currently do not address CI/CD pipeline configuration. This is independent of the prepare-validate-deploy workflow and should be a standalone skill.

Start by reviewing the existing azure-prepare and azure-deploy skills and the referenced azd pipeline config workflow. Use the issue’s coverage list to assess the standalone pipeline-config scope across GitHub Actions, Azure DevOps, credentials, secrets, environments, and approvals. Done means the skill addresses each listed CI/CD scenario without duplicating the prepare-validate-deploy workflow.

Written by the indexing model from the issue text.