Fix intro greeting tone and load history on return visits

- soul-demo.c: rewrite intro system prompt — remove 'to see if you are real'
  and 'say something true about who you are' which were producing alive/sentient
  language. New prompt: friendly hello, ask how they're doing, explicitly no
  alive/sentient/experiencing anything lines.
- chat-widget: Turnstile callback now replays existing session history instead
  of always firing a new greeting — returning users within the same day see
  their conversation, not a duplicate hello.

src/js/chat-widget.el

@@ -385,7 +385,14 @@ fn main() -> Void {
                   if (msgs)     msgs.style.display     = 'flex';
                   if (inputRow) inputRow.style.display = 'flex';
                   updateCountdown();
-                  _sendIntroGreeting();
+                  // Replay existing history if present; only greet fresh sessions
+                  if (session.messages && session.messages.length > 0) {
+                    if (msgs && msgs.children.length === 0) {
+                      session.messages.forEach(function(m) { addMsg(m.role, m.text, true); });
+                    }
+                  } else {
+                    _sendIntroGreeting();
+                  }
                   var inp = document.getElementById('neuron-demo-text');
                   if (inp) inp.focus();
                 },