feat(chat): EA2-backed 1:1 messaging surface

New Chat scene at the Chat top-bar tab. Threads and messages are stored
as flow docs (kind=definition, source_context=EA2_CHAT_THREAD / _MSG)
linked by defines edges with role=next — same proven contract as wizard
steps. Persona directory lets the signed-in operator open a thread with
the CEO / HR / IT personas.

- src/lib/chatApi.ts: listThreads / createThread / listMessages / sendMessage
- src/scenes/Chat.tsx: sidebar (threads + new-thread picker) + main pane
  (header, bubbles, composer with Enter-to-send / Shift-Enter newline)
- src/index.css: chat-scene grid + bubble + composer styles
- src/App.tsx: Chat tab + scene route

src/App.tsx

@@ -8,10 +8,11 @@ import Wizard from "./scenes/Wizard";
 import Settings from "./scenes/Settings";
 import Login from "./scenes/Login";
 import SsoCallback from "./scenes/SsoCallback";
+import Chat from "./scenes/Chat";
 import CommandBar from "./components/CommandBar";
 import Toaster from "./components/Toaster";
 import Console from "./components/Console";
-import { Cmd, Home, Layers, HistoryIcon, Pulse, Refresh, Branch, Cog, User, Sun, Moon } from "./components/icons";
+import { Cmd, Home, Layers, HistoryIcon, Pulse, Refresh, Branch, Cog, User, Sun, Moon, Bot } from "./components/icons";
 import { liveMeta } from "./data/scenarios";
 
 export default function App() {
@@ -71,6 +72,9 @@ export default function App() {
             <button role="tab" aria-selected={scene === "studio"} className={`tab${scene === "studio" ? " tab-sel" : ""}`} onClick={() => setScene("studio")}>
               <Branch size={13} /> Studio
             </button>
+            <button role="tab" aria-selected={scene === "chat"} className={`tab${scene === "chat" ? " tab-sel" : ""}`} onClick={() => setScene("chat")}>
+              <Bot size={13} /> Chat
+            </button>
             <button role="tab" aria-selected={scene === "settings"} className={`tab${scene === "settings" ? " tab-sel" : ""}`} onClick={() => setScene("settings")}>
               <Cog size={13} /> Settings
             </button>
@@ -149,6 +153,7 @@ export default function App() {
         {scene === "mission" && <MissionControl />}
         {scene === "history" && <RunHistory />}
         {scene === "studio" && <Wizard />}
+        {scene === "chat" && <Chat />}
         {scene === "settings" && <Settings />}
       </div>