feat(orchestration): add sub-workflow composition as reusable blocks (#2143)

[mrveiss]

Summary

• SubWorkflowExecutor enables workflows to call other workflows as steps
• Variable mapping from parent to child via input_mapping with VariableResolver
• Child results stored as StepOutput for parent variable piping
• Max nesting depth guard (5) prevents infinite recursion
• is_sub_workflow_step() predicate routes in WorkflowExecutor
• Backward-compatible: workflow_fetcher is optional
• 28 tests across 5 test classes

Closes #2143

Test plan

• Step detection: valid, wrong type, missing/empty workflow_id
• Extraction: minimal, full mapping, validation errors
• Basic execution: success, failure, not-found, correct ID
• Variable mapping: literals, ${steps...}, unresolvable, empty, output_key
• Max depth: at limit (raises), below limit (executes)
• flake8 passes

🤖 Generated with Claude Code

[github-actions]

✅ SSOT Configuration Compliance: Passing

🎉 No hardcoded values detected that have SSOT config equivalents!