feat(workflow): carry skillName through the WorkflowStep round-trip (U1)

Add skillName to WorkflowStep and WorkflowStepInput, and round-trip it through
nodeToStepInput / stepInputToNode so a skill-executor node's skill is available
to the step session. Honors the compiler INVERSION CONTRACT (parity test).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/core/src/types.ts

@@ -626,6 +626,11 @@ export interface WorkflowStep {
   prompt: string;
   /** Tool set available to prompt-mode workflow agents. Defaults to readonly. */
   toolMode?: WorkflowStepToolMode;
+  /** Name of a skill to load into this step's session (e.g.
+   *  "compound-engineering:ce-work"). When set, the step session loads the named
+   *  skill (discovery + selection) and the engine injects the Fusion workflow-step
+   *  conventions preamble. Only meaningful for skill-executor graph nodes. */
+  skillName?: string;
   /** Name of a script from project settings `scripts` map to execute (required when mode is "script") */
   scriptName?: string;
   /** Whether this step is available for selection on new tasks */
@@ -746,6 +751,9 @@ export interface WorkflowStepInput {
   prompt?: string;
   /** Tool set available to prompt-mode workflow agents. Defaults to readonly. */
   toolMode?: WorkflowStepToolMode;
+  /** Name of a skill to load into this step's session (e.g.
+   *  "compound-engineering:ce-work"). See `WorkflowStep.skillName`. */
+  skillName?: string;
   /** Script name from project settings (required when mode is "script").
    *  Must reference a named script in `settings.scripts` — no raw commands. */
   scriptName?: string;

packages/core/src/workflow-compiler.ts

@@ -231,8 +231,9 @@ function defaultGateMode(node: WorkflowIrNode, mode: "prompt" | "script"): Workf
  * its exact inverse is `stepInputToNode` in `workflow-steps-to-ir.ts`. Parity is
  * pinned by `__tests__/workflow-steps-to-ir.test.ts` over exactly the
  * compiler-visible fields: name / mode / phase / gateMode / prompt / scriptName /
- * toolMode / modelProvider / modelId. `enabled` / `defaultOn` / `templateId` are
- * NOT compiler-visible and are handled by migration policy, not the converter.
+ * toolMode / skillName / modelProvider / modelId. `enabled` / `defaultOn` /
+ * `templateId` are NOT compiler-visible and are handled by migration policy, not
+ * the converter.
  *
  * INVERSION CONTRACT: when you add a field here, extend `stepInputToNode` (and
  * the parity test) in `workflow-steps-to-ir.ts` to keep the round-trip exact.
@@ -255,6 +256,10 @@ function nodeToStepInput(node: WorkflowIrNode, phase: "pre-merge" | "post-merge"
   } else {
     input.prompt = configString(node, "prompt") ?? "";
     input.toolMode = node.config?.toolMode === "coding" ? "coding" : "readonly";
+    // Carry the node's skill name so the step session can load it (U1). Only
+    // present on skill-executor nodes; omitted otherwise to keep round-trip exact.
+    const skillName = configString(node, "skillName");
+    if (skillName) input.skillName = skillName;
     const provider = configString(node, "modelProvider");
     const modelId = configString(node, "modelId");
     if (provider && modelId) {

packages/core/src/workflow-steps-to-ir.ts

@@ -12,10 +12,10 @@ import { parseWorkflowIr } from "./workflow-ir.js";
  *   compileWorkflowToSteps(stepsToWorkflowIr(steps, name)) ≡ steps
  *
  * over exactly the compiler-visible fields: name / mode / phase / gateMode /
- * prompt / scriptName / toolMode / modelProvider / modelId. `enabled` /
- * `defaultOn` / `templateId` / `migratedFragmentId` are NOT compiler-visible and
- * are handled by migration policy (KTD-3), not by this converter. Parity is
- * pinned by `__tests__/workflow-steps-to-ir.test.ts`.
+ * prompt / scriptName / toolMode / skillName / modelProvider / modelId.
+ * `enabled` / `defaultOn` / `templateId` / `migratedFragmentId` are NOT
+ * compiler-visible and are handled by migration policy (KTD-3), not by this
+ * converter. Parity is pinned by `__tests__/workflow-steps-to-ir.test.ts`.
  *
  * INVERSION CONTRACT: when a compiler-visible field is added to `nodeToStepInput`
  * (see the contract comment there), extend `stepInputToNode` below and the parity
@@ -68,6 +68,9 @@ function stepInputToNode(step: WorkflowStep, id: string): WorkflowIrNode {
   // prompt mode
   config.prompt = step.prompt ?? "";
   config.toolMode = step.toolMode === "coding" ? "coding" : "readonly";
+  // Skill name round-trips when set (U1). The compiler reads it back via
+  // configString(node, "skillName"), so this keeps the inverse exact.
+  if (step.skillName) config.skillName = step.skillName;
   // Model overrides only round-trip when BOTH are present (compiler requirement).
   if (step.modelProvider && step.modelId) {
     config.modelProvider = step.modelProvider;