Add optional cross-AI delegation step that lets execute-phase delegate
plans to external AI runtimes via stdin-based prompt delivery. Activated
by --cross-ai flag, plan frontmatter cross_ai: true, or config key
workflow.cross_ai_execution. Adds 3 config keys, template defaults,
and 18 tests.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
