Add instance experimental setting for isolated workspaces

Introduce a singleton instance_settings store and experimental settings API, add the Experimental instance settings page, and gate execution workspace behavior behind the new enableIsolatedWorkspaces flag.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

server/src/__tests__/execution-workspace-policy.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, it } from "vitest";
 import {
   buildExecutionWorkspaceAdapterConfig,
   defaultIssueExecutionWorkspaceSettingsForProject,
+  gateProjectExecutionWorkspacePolicy,
   parseIssueExecutionWorkspaceSettings,
   parseProjectExecutionWorkspacePolicy,
   resolveExecutionWorkspaceMode,
@@ -140,4 +141,19 @@ describe("execution workspace policy helpers", () => {
       mode: "shared_workspace",
     });
   });
+
+  it("disables project execution workspace policy when the instance flag is off", () => {
+    expect(
+      gateProjectExecutionWorkspacePolicy(
+        { enabled: true, defaultMode: "isolated_workspace" },
+        false,
+      ),
+    ).toBeNull();
+    expect(
+      gateProjectExecutionWorkspacePolicy(
+        { enabled: true, defaultMode: "isolated_workspace" },
+        true,
+      ),
+    ).toEqual({ enabled: true, defaultMode: "isolated_workspace" });
+  });
 });