feat: derive structured foreach runtime context

src/wf_core/context_contracts.py

@@ -13,6 +13,7 @@ LINEAGE_ID_CONTEXT_KEY = "lineage_id"
 PARENT_LINEAGE_ID_CONTEXT_KEY = "parent_lineage_id"
 LOOP_ITEM_CONTEXT_KEY = "loop_item"
 LOOP_INDEX_CONTEXT_KEY = "loop_index"
+FOREACH_CONTEXT_KEY = "foreach"
 
 
 @dataclass(frozen=True, slots=True)
@@ -57,6 +58,7 @@ STANDARD_CONTEXT_FIELD_NAMES = frozenset(
 RESERVED_CONTEXT_KEYS = STANDARD_CONTEXT_FIELD_NAMES | {
     LOOP_ITEM_CONTEXT_KEY,
     LOOP_INDEX_CONTEXT_KEY,
+    FOREACH_CONTEXT_KEY,
 }