Introduce the `optional-group` node kind and `WorkflowOptionalGroupConfig`
(defaultOn + single-pass template subgraph), validated by mirroring the loop
template rules: non-empty template, single entry/exit, no nested groups, no
rework edges (single-pass guarantee), no template/top-level id collisions,
boolean defaultOn. Foundation for graph-native optional steps; additive and
inert until producers/consumers land in later units. Includes the plan.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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