fix(media): require staged sandbox media refs

src/agents/sandbox-media-paths.test.ts

@@ -49,18 +49,24 @@ describe("createSandboxBridgeReadFile", () => {
       relativePath: filePath,
       containerPath: `/sandbox/${filePath}`,
     }));
+    const stat = vi.fn(async () => ({ type: "file", size: 1, mtimeMs: 1 }));
 
     const resolved = await resolveSandboxedBridgeMediaPath({
       sandbox: {
         root: "/tmp/sandbox-root",
         bridge: {
           resolvePath,
+          stat,
         } as unknown as SandboxFsBridge,
       },
       mediaPath: "media://inbound/photo.png",
       inboundFallbackDir: "media/inbound",
     });
 
+    expect(stat).toHaveBeenCalledWith({
+      filePath: "media/inbound/photo.png",
+      cwd: "/tmp/sandbox-root",
+    });
     expect(resolvePath).toHaveBeenCalledWith({
       filePath: "media/inbound/photo.png",
       cwd: "/tmp/sandbox-root",
@@ -70,4 +76,22 @@ describe("createSandboxBridgeReadFile", () => {
       rewrittenFrom: "media://inbound/photo.png",
     });
   });
+
+  it("rejects missing staged inbound media URIs before direct sandbox resolution", async () => {
+    const resolvePath = vi.fn();
+    await expect(
+      resolveSandboxedBridgeMediaPath({
+        sandbox: {
+          root: "/tmp/sandbox-root",
+          bridge: {
+            resolvePath,
+            stat: vi.fn(async () => null),
+          } as unknown as SandboxFsBridge,
+        },
+        mediaPath: "media://inbound/missing.png",
+        inboundFallbackDir: "media/inbound",
+      }),
+    ).rejects.toThrow("Sandbox media reference is not staged: media://inbound/missing.png");
+    expect(resolvePath).not.toHaveBeenCalled();
+  });
 });

src/agents/sandbox-media-paths.ts

@@ -31,6 +31,15 @@ export async function resolveSandboxedBridgeMediaPath(params: {
     : { resolved: params.mediaPath };
   const filePath = normalizeFileUrl(mediaPathInfo.resolved);
   const rewrittenFrom = mediaPathInfo.rewrittenFrom;
+  if (rewrittenFrom) {
+    const stat = await params.sandbox.bridge.stat({
+      filePath,
+      cwd: params.sandbox.root,
+    });
+    if (!stat) {
+      throw new Error(`Sandbox media reference is not staged: ${rewrittenFrom}`);
+    }
+  }
   const enforceWorkspaceBoundary = async (hostPath: string) => {
     if (!params.sandbox.workspaceOnly) {
       return;