Loads a task's selected workflow, runs the graph with injected legacy seams
(execute/review/merge) and a custom-node runner, and maps the terminal outcome
to completed/failed/fell-back. Any interpreter-level error falls back so the
caller can run the legacy pipeline — a task is never stranded. Covered with
fake seams: lifecycle ordering, failure routing, gate blocking, fallback
reasons, diagnostics isolation. Includes the interpreter-cutover plan doc.
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