feat(workflow): run ce-work for compound-engineering execute step (U4)

Swap the compound-engineering workflow's execute seam for a
compound-engineering:ce-work skill node in coding mode, so the
implementation stage actually runs the CE way (R1). Add tests asserting
the ce-work skill executor and coding toolMode.

Part of the compound-engineering workflow integration plan
(docs/plans/2026-06-13-002).

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

.changeset/feat-compound-engineering-workflow-integration.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": minor
+---
+
+Make the built-in compound-engineering workflow run the CE way end-to-end. The execute stage now invokes the `compound-engineering:ce-work` skill in coding mode instead of the generic executor prompt, so implementation follows the compound-engineering workflow. (Further stages — CE commit/PR merge flow, human-in-the-loop planning questions, and subagent enablement — land in follow-up commits on this feature.)

packages/core/src/__tests__/builtin-workflows.test.ts

@@ -268,11 +268,26 @@ describe("built-in workflows", () => {
   it("compound-engineering compiles its skill nodes to steps", () => {
     const ce = getBuiltinWorkflow("builtin:compound-engineering")!;
     const steps = compileWorkflowToSteps(ce.ir);
-    // plan + code-review (pre-merge) + document (post-merge) — seams are skipped.
-    expect(steps.length).toBeGreaterThanOrEqual(3);
+    // plan + execute (ce-work) + code-review (pre-merge) + document (post-merge)
+    // — review/merge seams are skipped.
+    expect(steps.length).toBeGreaterThanOrEqual(4);
     expect(steps.some((s) => s.name === "Plan")).toBe(true);
   });
 
+  it("compound-engineering runs ce-work for the execute step in coding mode", () => {
+    const ce = getBuiltinWorkflow("builtin:compound-engineering")!;
+    // The IR node declares the ce-work skill executor (engine wraps the prompt
+    // with the invoke-skill preamble on the graph-interpreter path).
+    const executeNode = ce.ir.nodes.find((n) => n.id === "execute");
+    expect(executeNode?.config?.executor).toBe("skill");
+    expect(executeNode?.config?.skillName).toBe("compound-engineering:ce-work");
+    // The compiled step runs in coding mode so write/spawn tools are available.
+    const steps = compileWorkflowToSteps(ce.ir);
+    const execute = steps.find((s) => s.name === "Execute");
+    expect(execute).toBeDefined();
+    expect(execute!.toolMode).toBe("coding");
+  });
+
   describe("store integration", () => {
     const harness = createTaskStoreTestHarness();
     let store: ReturnType<typeof harness.store>;

packages/core/src/builtin-workflows.ts

@@ -165,7 +165,20 @@ export const BUILTIN_WORKFLOWS: WorkflowDefinition[] = [
           prompt: "Produce a short implementation plan for this task before any code is written.",
         },
       },
-      { id: "execute", kind: "prompt", config: builtinPromptConfig("execute", "Execute") },
+      {
+        id: "execute",
+        kind: "prompt",
+        config: {
+          name: "Execute",
+          executor: "skill",
+          skillName: "compound-engineering:ce-work",
+          // Coding mode so the step has write + spawn tools (readonly is the
+          // default and would strip them). ce-work does the implementation the
+          // CE way instead of the generic executor seam.
+          toolMode: "coding",
+          prompt: "Execute the plan for this task, following existing patterns and maintaining quality throughout.",
+        },
+      },
       { id: "review", kind: "prompt", config: builtinPromptConfig("review", "Review") },
       {
         id: "code-review",