FN-6035 modeled the merge lifecycle as a branching subgraph of merge/
retry/branch-group primitives instead of a single `merge` seam, but the
linear workflow compiler still tried to lower those nodes and rejected
merge-gate's fan-out — so task creation failed with "node 'merge-gate'
branches into 2 edges — graphs with branches require the workflow
interpreter (deferred)".
Treat the merge-region primitive kinds (merge-gate, merge-attempt,
manual-merge-hold, retry-backoff, recovery-router,
branch-group-member-integration, branch-group-promotion, pr-merge) as an
engine-owned terminal boundary: exempt from the single-edge linearity
rule and never lowered to a step. Linear-prefix workflows compile to
their pre-merge step list again.
Also refresh the stale builtin-workflows assertions that referenced the
removed `merge` seam node.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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