feat(core): column agent IR schema, validation, and effective-agent resolver

U1+U2 of the column-agent plan: WorkflowColumnAgent on WorkflowIrColumn
(defer/override), template-subgraph column validation, v2-only-feature
registration, plugin-sdk type parity, and the shared core resolver with
instanceNodeId format ownership moved to core.

packages/plugin-sdk/src/index.ts

@@ -109,6 +109,10 @@ export type {
   WorkflowIrNode,
   WorkflowIrEdge,
   WorkflowIrNodeKind,
+  // Columns + per-column permanent-agent binding (column-agent plan KTD-1, R12).
+  WorkflowIrColumn,
+  WorkflowIrColumnTrait,
+  WorkflowColumnAgent,
   // Foreach / artifacts / custom fields (step inversion).
   WorkflowForeachConfig,
   WorkflowIrArtifact,