feat(FN-4639): complete Step 4 — add sandbox prompt override resolver

Fusion-Task-Id: FN-4639
Fusion-Task-Lineage: 40745d49-dd20-4c51-87e3-42062417f788

packages/core/src/sandbox-prompt-override.ts

@@ -0,0 +1,34 @@
+import type { SandboxBackendName, SandboxProjectSettings } from "./types.js";
+
+/**
+ * Sandbox backend override parsing for per-task PROMPT.md content (FN-4639; design from FN-4635).
+ *
+ * The engine can call these pure helpers to resolve the effective backend with precedence:
+ * PROMPT override (`**Sandbox:** <backend>`) > project settings > default native backend.
+ */
+const SANDBOX_OVERRIDE_RE = /^\*\*Sandbox:\*\*\s*(native|sandbox-exec|bubblewrap|docker|podman|custom)\s*$/im;
+
+export function parseSandboxPromptOverride(prompt: string | undefined): SandboxBackendName | undefined {
+  if (!prompt) {
+    return undefined;
+  }
+  const match = prompt.match(SANDBOX_OVERRIDE_RE);
+  return match ? (match[1] as SandboxBackendName) : undefined;
+}
+
+export function resolveSandboxBackend(
+  settings: { sandbox?: SandboxProjectSettings } | undefined,
+  prompt: string | undefined,
+): { backend: SandboxBackendName; source: "default" | "project" | "prompt" } {
+  const promptOverride = parseSandboxPromptOverride(prompt);
+  if (promptOverride) {
+    return { backend: promptOverride, source: "prompt" };
+  }
+
+  const projectBackend = settings?.sandbox?.backend;
+  if (projectBackend) {
+    return { backend: projectBackend, source: "project" };
+  }
+
+  return { backend: "native", source: "default" };
+}