PAP-87: add enableStreamlinedLeftNavigation experimental flag (plumbing)

Plumbs the boolean experimental flag end-to-end, default OFF. No nav
behavior yet — foundation for the other PAP-80 IA children to branch on.

- shared types/validators: add enableStreamlinedLeftNavigation (patch
  schema is .partial()-derived, so covered automatically)
- server: default it in normalizeExperimentalSettings (both paths)
- ui: "Streamlined Left Navigation Bar" ToggleSwitch on the experimental
  settings page, following the enableIsolatedWorkspaces pattern
- update instance-settings service test for the new field

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

packages/shared/src/types/instance.ts

@@ -29,6 +29,7 @@ export interface InstanceGeneralSettings {
 export interface InstanceExperimentalSettings {
   enableEnvironments: boolean;
   enableIsolatedWorkspaces: boolean;
+  enableStreamlinedLeftNavigation: boolean;
   enableIssuePlanDecompositions: boolean;
   enableCloudSync: boolean;
   autoRestartDevServerWhenIdle: boolean;

packages/shared/src/validators/instance.ts

@@ -38,6 +38,7 @@ export const patchInstanceGeneralSettingsSchema = instanceGeneralSettingsSchema.
 export const instanceExperimentalSettingsSchema = z.object({
   enableEnvironments: z.boolean().default(false),
   enableIsolatedWorkspaces: z.boolean().default(false),
+  enableStreamlinedLeftNavigation: z.boolean().default(false),
   enableIssuePlanDecompositions: z.boolean().default(false),
   enableCloudSync: z.boolean().default(false),
   autoRestartDevServerWhenIdle: z.boolean().default(false),

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

@@ -15,6 +15,7 @@ describe("instance settings service", () => {
     })).toEqual({
       enableEnvironments: true,
       enableIsolatedWorkspaces: true,
+      enableStreamlinedLeftNavigation: false,
       enableIssuePlanDecompositions: true,
       enableCloudSync: true,
       autoRestartDevServerWhenIdle: true,

server/src/services/instance-settings.ts

@@ -43,6 +43,7 @@ export function normalizeExperimentalSettings(raw: unknown): InstanceExperimenta
     return {
       enableEnvironments: parsed.data.enableEnvironments ?? false,
       enableIsolatedWorkspaces: parsed.data.enableIsolatedWorkspaces ?? false,
+      enableStreamlinedLeftNavigation: parsed.data.enableStreamlinedLeftNavigation ?? false,
       enableIssuePlanDecompositions: parsed.data.enableIssuePlanDecompositions ?? false,
       enableCloudSync: parsed.data.enableCloudSync ?? false,
       autoRestartDevServerWhenIdle: parsed.data.autoRestartDevServerWhenIdle ?? false,
@@ -55,6 +56,7 @@ export function normalizeExperimentalSettings(raw: unknown): InstanceExperimenta
   return {
     enableEnvironments: false,
     enableIsolatedWorkspaces: false,
+    enableStreamlinedLeftNavigation: false,
     enableIssuePlanDecompositions: false,
     enableCloudSync: false,
     autoRestartDevServerWhenIdle: false,

ui/src/pages/InstanceExperimentalSettings.tsx

@@ -205,6 +205,8 @@ export function InstanceExperimentalSettings() {
 
   const enableEnvironments = experimentalQuery.data?.enableEnvironments === true;
   const enableIsolatedWorkspaces = experimentalQuery.data?.enableIsolatedWorkspaces === true;
+  const enableStreamlinedLeftNavigation =
+    experimentalQuery.data?.enableStreamlinedLeftNavigation === true;
   const enableIssuePlanDecompositions =
     experimentalQuery.data?.enableIssuePlanDecompositions === true;
   const enableCloudSync = experimentalQuery.data?.enableCloudSync === true;
@@ -301,6 +303,28 @@ export function InstanceExperimentalSettings() {
         </div>
       </section>
 
+      <section className="rounded-xl border border-border bg-card p-5">
+        <div className="flex items-start justify-between gap-4">
+          <div className="space-y-1.5">
+            <h2 className="text-sm font-semibold">Streamlined Left Navigation Bar</h2>
+            <p className="max-w-2xl text-sm text-muted-foreground">
+              Reduces the maximum number of items in the left navigation bar — nests Projects under Work with a
+              dedicated Projects page, and shows only active agents (max 5 recently-active) in the sidebar.
+            </p>
+          </div>
+          <ToggleSwitch
+            checked={enableStreamlinedLeftNavigation}
+            onCheckedChange={() =>
+              toggleMutation.mutate({
+                enableStreamlinedLeftNavigation: !enableStreamlinedLeftNavigation,
+              })
+            }
+            disabled={toggleMutation.isPending}
+            aria-label="Toggle streamlined left navigation experimental setting"
+          />
+        </div>
+      </section>
+
       <section className="rounded-xl border border-border bg-card p-5">
         <div className="flex items-start justify-between gap-4">
           <div className="space-y-1.5">