feat(decisions): gate Decisions sidebar behind experimental flag (PAP-13481)

Add an `enableDecisions` instance experimental setting (default off), gate the
Decisions sidebar item on it, and move Decisions below Inbox in the sidebar.
Adds a toggle card on the Experimental settings page. Includes parity tests
for the validator default/patch, service normalization, the settings toggle,
and updates the full-object normalization assertion.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/src/__tests__/instance-settings-service.test.ts

@@ -32,6 +32,7 @@ describe("instance settings service", () => {
       enableTaskWatchdogs: true,
       enableCloudSync: true,
       enableBuiltInAgents: true,
+      enableDecisions: false,
       enableGoalsSidebarLink: true,
       enableServerInfoDebugView: true,
       autoRestartDevServerWhenIdle: true,
@@ -76,6 +77,14 @@ describe("instance settings service", () => {
     ).toBe(false);
   });
 
+  it("defaults enableDecisions to false for empty and legacy stored settings", () => {
+    expect(normalizeExperimentalSettings(undefined).enableDecisions).toBe(false);
+    expect(normalizeExperimentalSettings({}).enableDecisions).toBe(false);
+    expect(
+      normalizeExperimentalSettings({ enableStreamlinedLeftNavigation: true }).enableDecisions,
+    ).toBe(false);
+  });
+
   it("defaults workspace branch repair settings to true for empty and legacy stored settings", () => {
     expect(normalizeExperimentalSettings(undefined).enableWorkspaceBranchReconcileForward).toBe(true);
     expect(normalizeExperimentalSettings({}).enableWorkspaceBranchReconcileForward).toBe(true);

server/src/services/instance-settings.ts

@@ -56,6 +56,7 @@ export function normalizeExperimentalSettings(raw: unknown): InstanceExperimenta
       enableCloudSync: parsed.data.enableCloudSync ?? false,
       enableExternalObjects: parsed.data.enableExternalObjects ?? false,
       enableBuiltInAgents: parsed.data.enableBuiltInAgents ?? false,
+      enableDecisions: parsed.data.enableDecisions ?? false,
       enableGoalsSidebarLink: parsed.data.enableGoalsSidebarLink ?? false,
       enableServerInfoDebugView: parsed.data.enableServerInfoDebugView ?? false,
       autoRestartDevServerWhenIdle: parsed.data.autoRestartDevServerWhenIdle ?? false,
@@ -81,6 +82,7 @@ export function normalizeExperimentalSettings(raw: unknown): InstanceExperimenta
     enableCloudSync: false,
     enableExternalObjects: false,
     enableBuiltInAgents: false,
+    enableDecisions: false,
     enableGoalsSidebarLink: false,
     enableServerInfoDebugView: false,
     autoRestartDevServerWhenIdle: false,