fix(tui): gate background shortcut by capability (#32837)

packages/opencode/src/server/routes/instance/httpapi/groups/experimental.ts

@@ -25,6 +25,10 @@ const ConsoleStateResponse = Schema.Struct({
   switchableOrgCount: NonNegativeInt,
 }).annotate({ identifier: "ConsoleState" })
 
+const CapabilitiesResponse = Schema.Struct({
+  backgroundSubagents: Schema.Boolean,
+}).annotate({ identifier: "ExperimentalCapabilities" })
+
 const ConsoleOrgOption = Schema.Struct({
   accountID: Schema.String,
   accountEmail: Schema.String,
@@ -84,6 +88,7 @@ export const SessionListQuery = Schema.Struct({
 })
 
 export const ExperimentalPaths = {
+  capabilities: "/experimental/capabilities",
   console: "/experimental/console",
   consoleOrgs: "/experimental/console/orgs",
   consoleSwitch: "/experimental/console/switch",
@@ -100,6 +105,16 @@ export const ExperimentalApi = HttpApi.make("experimental")
   .add(
     HttpApiGroup.make("experimental")
       .add(
+        HttpApiEndpoint.get("capabilities", ExperimentalPaths.capabilities, {
+          query: WorkspaceRoutingQuery,
+          success: described(CapabilitiesResponse, "Experimental capabilities"),
+        }).annotateMerge(
+          OpenApi.annotations({
+            identifier: "experimental.capabilities.get",
+            summary: "Get experimental capabilities",
+            description: "Get experimental features enabled on the OpenCode server.",
+          }),
+        ),
         HttpApiEndpoint.get("console", ExperimentalPaths.console, {
           query: WorkspaceRoutingQuery,
           success: described(ConsoleStateResponse, "Active Console provider metadata"),