feat(tui): event-derived ordering + retire Kotlin TUI

Rebuild the Go TUI transcript from the event stream instead of
optimistic local echoes. ChatTurnEvent now maps to ServerMessage.ChatTurn
(chat.turn); the TUI auto-vivifies sessions and renders user+router turns
from events. SessionStarted is de-special-cased into SessionAnnounced
(still carries workflowId), dropping the router.response shadow and
optimistic echoes.

Delete the unused Kotlin TUI (apps/tui) and add a kotlinx<->Go
golden-fixture test to lock the wire protocol. Gitignore server runtime
logs.

apps/tui-go/internal/app/update.go

@@ -545,15 +545,14 @@ func (m Model) submit() (tea.Model, tea.Cmd) {
 		if text == "" {
 			return m, nil
 		}
-		// New optimistic chat session.
+		// New chat session. The client owns the id, so create the entry locally and
+		// focus it; the USER/ROUTER turns arrive as chat.turn events (no optimistic echo).
 		id := newSessionID()
-		m.sessions = append(m.sessions, Session{
-			ID: id, Status: "STARTING", WorkflowID: "chat", Name: "chat",
-			LastEventAt: nowMillis(),
-		})
+		s := m.ensureSession(id)
+		s.WorkflowID = "chat"
+		s.Name = "chat"
 		m.selectedID = id
 		m.sessionEntered = true
-		m.appendRouter(id, RouterEntry{"user", text})
 		m.client.Send(protocol.StartChatSession(id, text))
 		m.clearInput()
 		return m, nil
@@ -569,7 +568,7 @@ func (m Model) submit() (tea.Model, tea.Cmd) {
 			hist = hist[len(hist)-50:]
 		}
 		m.history[sid] = hist
-		m.appendRouter(sid, RouterEntry{"user", text})
+		// No optimistic echo — the USER turn arrives as a chat.turn event.
 		m.client.Send(protocol.ChatInput(sid, text, m.chatMode))
 		m.clearInput()
 		return m, nil