fix(gui): add retry backoff for agent connections

Agent connections were using retryMs=0, causing a tight loop when the
/connection file read failed (e.g., toolsConn nil). The loop was:
  connection fails → disconnected signal → refreshSessions →
  reconcileAgentConnections → new connection → fails immediately → repeat

Fix by:
- Add maxRetries parameter to NinePConnection::start()
- Use 5s retry interval with max 3 retries for agent connections
- Reset retry count on successful connection

gui/ninepconnection.cpp

@@ -12,12 +12,14 @@ NinePConnection::~NinePConnection()
     stop();
 }
 
-void NinePConnection::start(const QString &program, const QStringList &args, int retryMs, bool probe)
+void NinePConnection::start(const QString &program, const QStringList &args, int retryMs, bool probe, int maxRetries)
 {
     m_program = program;
     m_args = args;
     m_retryMs = retryMs;
     m_probe = probe;
+    m_maxRetries = maxRetries;
+    m_retryCount = 0;
     m_stopRequested = false;
     m_openMarkerSeen = false;
     m_retryTimer.stop();
@@ -91,17 +93,21 @@ void NinePConnection::onFinished(int exitCode, QProcess::ExitStatus)
     if (!m_process) return;
     m_process->deleteLater();
     m_process = nullptr;
+    // Successful probe that was connected: schedule re-probe (reset retry count).
     if (m_probe && m_state == Connected && m_openMarkerSeen && exitCode == 0 && !m_stopRequested) {
+        m_retryCount = 0;
         if (m_retryMs > 0)
             m_retryTimer.start(m_retryMs);
         else
             startProcess(); // immediate re-probe
         return;
     }
-    if (m_stopRequested || m_retryMs <= 0) {
+    // No retry configured, or stop requested, or max retries exceeded.
+    if (m_stopRequested || m_retryMs <= 0 || (m_maxRetries > 0 && m_retryCount >= m_maxRetries)) {
         setState(Disconnected);
         return;
     }
+    ++m_retryCount;
     setState(RetryWait);
     m_retryTimer.start(m_retryMs);
 }

gui/ninepconnection.h

@@ -16,7 +16,7 @@ public:
     explicit NinePConnection(QObject *parent = nullptr);
     ~NinePConnection() override;
 
-    void start(const QString &program, const QStringList &args, int retryMs = 4000, bool probe = false);
+    void start(const QString &program, const QStringList &args, int retryMs = 4000, bool probe = false, int maxRetries = 0);
     void stop();
     State state() const { return m_state; }
 
@@ -47,6 +47,8 @@ private:
     int m_retryMs = 4000;
     bool m_probe = false;
     bool m_openMarkerSeen = false;
+    int m_maxRetries = 0;    // 0 = unlimited
+    int m_retryCount = 0;
 };
 
 #endif // NINEPCONNECTION_H

gui/ollie9pclient.cpp

@@ -254,11 +254,10 @@ void Ollie9pClient::reconcileAgentConnections()
                 // rather than showing a transient red indicator.
                 refreshSessions();
             });
-            // No retry — reconcileAgentConnections is the authority for
-            // connection lifecycle. If the agent is still alive after refresh,
-            // reconcile will create a new connection.
+            // Retry with backoff (5s, max 3 retries) to avoid hammering the server
+            // when the connection file exists but returns an error (e.g., toolsConn nil).
             connection->start(ollie9pBin(), {"-a", serverAddr(), "read", "--open-marker",
-                "session/" + sid + "/agent/" + aid + "/connection"}, 0, true);
+                "session/" + sid + "/agent/" + aid + "/connection"}, 5000, true, 3);
         }
     }
     for (auto it = m_agentConnections.begin(); it != m_agentConnections.end();) {