Add use_pipeline step type for pipeline composition

[viamus]

Summary

• New use_pipeline step type that loads and executes another pipeline YAML as a sub-pipeline, enabling pipeline composition and reuse
• Supports inputs: mapping from parent context (with {{steps.xxx}} template resolution) and output merging back into the parent (with/without output_key, respects child outputs: section)
• Circular reference detection via AsyncLocal<ImmutableHashSet<string>> — safe across async/parallel boundaries
• Full optional: true support — child pipeline failure converts to Skipped
• 26 new tests covering parsing, validation, execution, input/output mapping, error handling, and edge cases

Example usage

steps:
  - name: "Prepare data"
    prompt: "..."
    output_key: data

  - name: "Run analysis"
    use_pipeline: ./pipelines/analysis.yml
    inputs:
      source: "{{steps.data}}"
    output_key: analysis_result

  - name: "Report"
    prompt: "Generate report from {{steps.analysis_result}}"

Test plan

• dotnet build compiles without errors
• dotnet test — all 137 tests pass (26 new)
• Manual: dotnet run --project CodeGenesis.Engine -- run-pipeline examples/use-pipeline/main.yml

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