feat(FN-4938): complete Step 2 — add helper and wire cleanup

Fusion-Task-Id: FN-4938
Fusion-Task-Lineage: 8b53a5da-1321-4796-9775-ff043efdcef3

packages/engine/src/worktree-desktop-artifacts.ts

@@ -0,0 +1,44 @@
+import { existsSync } from "node:fs";
+import { rm } from "node:fs/promises";
+import { resolve } from "node:path";
+
+export const DESKTOP_ARTIFACT_RELATIVE_PATHS = ["packages/desktop/dist", "packages/desktop/dist-electron"] as const;
+
+type CleanupLogger = {
+  log: (message: string) => void;
+  warn: (message: string) => void;
+};
+
+export async function removeDesktopBuildArtifacts(
+  worktreePath: string,
+  logger?: CleanupLogger,
+): Promise<{ removed: string[]; skipped: string[]; failures: Array<{ path: string; error: string }> }> {
+  const removed: string[] = [];
+  const skipped: string[] = [];
+  const failures: Array<{ path: string; error: string }> = [];
+
+  if (!worktreePath) {
+    logger?.warn?.("Desktop artifact cleanup skipped: missing worktree path");
+    return { removed, skipped, failures };
+  }
+
+  for (const relativePath of DESKTOP_ARTIFACT_RELATIVE_PATHS) {
+    const absolutePath = resolve(worktreePath, relativePath);
+    if (!existsSync(absolutePath)) {
+      skipped.push(relativePath);
+      continue;
+    }
+
+    try {
+      await rm(absolutePath, { recursive: true, force: true });
+      removed.push(relativePath);
+      logger?.log?.(`Removed desktop build artifact directory: ${relativePath}`);
+    } catch (error) {
+      const message = error instanceof Error ? error.message : String(error);
+      failures.push({ path: relativePath, error: message });
+      logger?.warn?.(`Failed to remove desktop build artifact directory ${relativePath}: ${message}`);
+    }
+  }
+
+  return { removed, skipped, failures };
+}