Add BETA tag: sidebar wordmark, boot splash, onboarding, README

Quiet gray chip, presentation-only — never in bundle or artifact names (updater safety).

surfaces/gui/src/components/Onboarding.tsx

@@ -119,7 +119,7 @@ export function Onboarding({ onDone }: { onDone: (next?: "work" | "gallery" | "a
         {step === 0 && (
           <section data-testid="ob-step-model" className="flex-1 min-h-0 flex flex-col">
             {/* Persistent header — stays put while the region below swaps (§39). */}
-            <h1 className="text-[19px] font-semibold">Welcome to OpenWorker</h1>
+            <h1 className="text-[19px] font-semibold">Welcome to OpenWorker<span className="beta-tag">BETA</span></h1>
             <p className="text-[13px] text-muted mt-0.5 mb-4">
               Pick a model provider to get started — OpenWorker runs on your own key, and your
               key and your data stay on this Mac.

surfaces/gui/src/components/Sidebar.tsx

@@ -1002,7 +1002,7 @@ export function Sidebar(props: Props) {
             <Icon name="sidebar" size={16} />
           </button>
         )}
-        <div className="brand-wordmark text-[15px]">OpenWorker</div>
+        <div className="brand-wordmark text-[15px]">OpenWorker<span className="beta-tag">BETA</span></div>
       </div>
 
       {/* New session: split button — primary starts the last-used persona; ▾ picks a specific one. */}