Add deterministic-tests workspace

Scripted multi-client harness against a real server (~110 scenario
tests, server-control, managed-websocket, test-runner). Wires the new
package into frontend/package.json workspaces and the lint script.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/deterministic-tests/src/tests/offline-update-both-then-delete-one.test.ts

@@ -0,0 +1,75 @@
+import type { AssertableState } from "../utils/assertable-state";
+import type { TestDefinition } from "../test-definition";
+
+export const offlineUpdateBothThenDeleteOneTest: TestDefinition = {
+    description:
+        "Client 0 goes offline, updates A.md and B.md, then deletes B.md. " +
+        "Client 1 updates B.md while Client 0 is offline. When Client 0 " +
+        "reconnects, A.md should have the update and B.md should be " +
+        "consistently resolved (delete wins).",
+    clients: 2,
+    steps: [
+        {
+            type: "create",
+            client: 0,
+            path: "A.md",
+            content: "A original"
+        },
+        {
+            type: "create",
+            client: 0,
+            path: "B.md",
+            content: "B original"
+        },
+        { type: "enable-sync", client: 0 },
+        { type: "enable-sync", client: 1 },
+        { type: "barrier" },
+        {
+            type: "assert-consistent",
+            verify: (s: AssertableState): void => {
+                s.assertContent("A.md", "A original").assertContent(
+                    "B.md",
+                    "B original"
+                );
+            }
+        },
+
+        { type: "disable-sync", client: 0 },
+
+        {
+            type: "update",
+            client: 0,
+            path: "A.md",
+            content: "A updated by client 0"
+        },
+        {
+            type: "update",
+            client: 0,
+            path: "B.md",
+            content: "B updated by client 0"
+        },
+
+        { type: "delete", client: 0, path: "B.md" },
+
+        {
+            type: "update",
+            client: 1,
+            path: "B.md",
+            content: "B updated by client 1"
+        },
+        { type: "sync", client: 1 },
+
+        { type: "enable-sync", client: 0 },
+        { type: "barrier" },
+
+        {
+            type: "assert-consistent",
+            verify: (s: AssertableState): void => {
+                s.assertContent(
+                    "A.md",
+                    "A updated by client 0"
+                ).assertFileNotExists("B.md");
+            }
+        }
+    ]
+};