refactor: explicit returns + void floating promises (T3.a typeaware sweep) (#756)

Cluster T3.a — mechanical fixes for consistent-return and no-floating-promises.

Files touched (15):
- server/shared/tool-call-display.ts — default throw in buildCanonicalDetailDisplay switch
- server/agent/prompt-attachments.ts — default throw in renderPromptAttachmentAsText switch
- server/workspace-directory.ts — default throw in getSortValue switch
- server/agent/mcp-server.ts — default throw in mcpCreateWorktreeInput switch
- server/terminal/terminal-session-controller.ts — default return in dispatch switch
- server/daemon-e2e/agent-configs.ts — default return false in isProviderAvailable switch
- server/agent/agent-manager.ts — void dispatchSessionEvent fire-and-forget call
- server/agent/mcp-shared.ts — void 4 notify() floating calls
- app/agent-status-bar.utils.ts — default throw in getStatusSelectorHint switch
- app/keyboard-shortcuts.ts — default throw in resolvePayload switch
- app/resolve-agent-form.ts — default throw in resolveAgentForm switch
- app/ui/dropdown-menu.tsx — void .then() + return undefined for promise/always-return
- app/use-agent-form-state.ts — void refreshSnapshot() floating call
- cli/worktree/create.ts — default throw in toDaemonCreateInput switch
- cli/onboard.ts — return process.exit(1) to satisfy consistent-return in catch

Deferred (outside 15-file cap or in EXCLUDE list):
- server/worktree-session.ts — getSortValue (same pattern, cap reached)
- Multiple files with other errors (unbound-method, unsafe-type-assertion) are separate clusters

packages/app/src/components/agent-status-bar.utils.ts

@@ -11,6 +11,8 @@ export function getStatusSelectorHint(selector: ExplainedStatusSelector): string
       return "Change model";
     case "mode":
       return "Change permission mode";
+    default:
+      throw new Error("unreachable");
   }
 }
 

packages/app/src/components/ui/dropdown-menu.tsx

@@ -445,7 +445,7 @@ export function DropdownMenuContent({
       setTriggerRect(null);
       setContentSize(null);
       setPosition(null);
-      return;
+      return undefined;
     }
 
     // Capture status bar height synchronously before async measurement.
@@ -454,8 +454,8 @@ export function DropdownMenuContent({
     const statusBarHeight = Platform.OS === "android" ? (StatusBar.currentHeight ?? 0) : 0;
     let cancelled = false;
 
-    measureElement(triggerRef.current).then((rect) => {
-      if (cancelled) return;
+    void measureElement(triggerRef.current).then((rect) => {
+      if (cancelled) return undefined;
       // On Android with statusBarTranslucent, measureInWindow returns coordinates
       // relative to below the status bar, but Modal content starts from screen top.
       // Add status bar height to align coordinate systems (same as react-native-popover-view).
@@ -463,7 +463,7 @@ export function DropdownMenuContent({
         ...rect,
         y: rect.y + statusBarHeight,
       });
-      return;
+      return undefined;
     });
 
     return () => {