dbus: emit ConfigChanged on agent switch, fix agent default override

- Add agent watcher goroutine that emits ConfigChanged(session, key, value)
  D-Bus signal when the per-session agent name changes
- Add ConfigChanged to introspection XML
- Fix CreateSession: only apply OLLIE_DEFAULT_AGENT when no agent arg was
  explicitly provided. Previously, passing agent=default would trigger the
  env override, making it impossible to select a specific agent from the GUI.

dbus.go

@@ -151,6 +151,19 @@ func (a *DBusAdapter) startWatcher(id string, sess *session.Session) {
 		}
 	}()
 
+	// Agent watcher — emits ConfigChanged when the agent name changes
+	go func() {
+		current := sess.Core.AgentName()
+		for {
+			next, ok := sess.Core.WaitChange(ctx, agent.WatchAgent, current)
+			if !ok {
+				return
+			}
+			current = next
+			a.conn.Emit(busPath, busIface+".ConfigChanged", id, "agent", current)
+		}
+	}()
+
 	// Chat watcher (polls log version)
 	go func() {
 		var lastLen int
@@ -751,5 +764,6 @@ var introspectXML = `<node>
     <signal name="ChatUpdated"><arg type="s" name="sessionID"/><arg type="x" name="offset"/><arg type="s" name="text"/></signal>
     <signal name="ProcessDetached"><arg type="s" name="sessionID"/></signal>
     <signal name="ProcessExited"><arg type="s" name="sessionID"/><arg type="i" name="pid"/><arg type="i" name="exitCode"/></signal>
+    <signal name="ConfigChanged"><arg type="s" name="sessionID"/><arg type="s" name="key"/><arg type="s" name="value"/></signal>
   </interface>
 </node>`

session/session.go

@@ -1060,7 +1060,7 @@ func (s *Manager) CreateSession(args []string) (string, error) {
 	name := ""
 	backendOverride := ""
 	modelOverride := ""
-	agentName := "default"
+	agentName := ""
 	cwd := ""
 	remoteTarget := ""
 	systemPromptOverride := ""
@@ -1091,9 +1091,11 @@ func (s *Manager) CreateSession(args []string) (string, error) {
 			return "", fmt.Errorf("unknown option %q (valid: name, backend, model, agent, cwd, remote, system_prompt)", k)
 		}
 	}
-	if agentName == "default" {
+	if agentName == "" {
 		if v := os.Getenv("OLLIE_DEFAULT_AGENT"); v != "" {
 			agentName = v
+		} else {
+			agentName = "default"
 		}
 	}