Real engine seams: execute delegates to the legacy implementation phase via a
completion interceptor that stops execute() at the implementation-complete
boundary (no double review/merge); review performs the in-review handoff; merge
resolves through ProjectEngine.onMerge over the same serialized merge queue
(wired via a late-bound setMergeRequester, mirroring setMergeEnqueuer). Custom
graph nodes run on the proven WorkflowStep machinery (readonly tool policy,
verdict parsing). Adds a 'planning' seam to the vocabulary (no-op for
pre-specified tasks; custom planning is a prompt node today).
Entry point: execute() routes graph-selected tasks through the runner when
experimentalFeatures.workflowGraphExecutor is on, with process-wide routing
claims (FN-4811 posture), duplicate-dispatch dropping, pre-run errors falling
back to legacy, and mid-run errors parking the task in review (never re-running
the implementation, never stranding the task).
Flag off by default: all 587 executor tests pass unchanged.
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