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/state/store.ts

@@ -6,7 +6,7 @@ import { buildLiveScenariosFromApi } from "../lib/buildScenarios";
 import { api, type ApiCall, type Actor } from "../lib/api";
 import type { ProcessScenario } from "../data/types";
 
-export type SceneId = "landing" | "mission" | "history" | "studio" | "settings" | "login" | "sso-callback";
+export type SceneId = "landing" | "mission" | "history" | "studio" | "settings" | "login" | "sso-callback" | "chat" | "agent" | "hub-procurement" | "hub-hr" | "hub-it" | "geo-attendance" | "explainer";
 export type DataMode = "snapshot" | "live";
 export type Theme = "dark" | "light";