feat(bot): every free-text message gets the agent's answer; every demo entry is a private copy

- Idle chat: all messages go to the agent (greetings, side-effect and substitute questions included); check-in
  signals are still recorded silently, quit intent keeps its rule reaction. Seen live: «какие будут побочки»
  was logged as a complaint («Спасибо, отметил»), «привет» got a canned phrase.
- Agent prompt: warm short replies to small talk; substitutes → only the doctor changes the drug.
- ЕМИАС demo persona button creates a private copy too (starting from consent + questionnaires, subject to
  the screening switch); the web app drops a stored session of the old shared demo account.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/session/SessionContext.tsx

@@ -13,7 +13,11 @@ interface SessionApi {
 const Ctx = createContext<SessionApi | null>(null);
 
 export function SessionProvider({ children }: { children: ReactNode }) {
-  const [session, setSession] = useState<Session | null>(() => sessionStore.load());
+  // The old shared demo account ("tok-andrey") is retired: every visitor now gets a private copy.
+  const [session, setSession] = useState<Session | null>(() => {
+    const s = sessionStore.load();
+    return s && s.token === "tok-andrey" ? null : s;
+  });
 
   const signIn = useCallback((auth: AuthResult) => {
     const next: Session = { token: auth.token, mode: auth.mode, displayName: auth.display_name };