fix(ui): treat an immediate ConPTY exit as a start failure

InteractiveSessionId is persisted before the ConPTY process spawns (1a988ff).
If `claude --session-id <guid>` exits immediately after launch (auth/network
hiccup, crash before the TUI starts), OnSessionProcessExited set HasExited
but never StartError, so ConPtyPaneViewModel.CanRetry (which requires
StartError) never offered Retry — every reopen just resumed the same dead
session id, permanently poisoning that task's interactive sessions.

Now a nonzero exit within 5s of the session becoming "running" is treated
as a died-at-startup failure and routed through the same StartError path
as a launch-time exception, so the pane shows the error banner + Retry.

Retry re-fetches the LaunchSpec via BuildForTaskAsync, which reuses the
same persisted InteractiveSessionId — correct for a transient failure
(fresh process, same id), but does not help a genuinely dead session id.
Clearing a dead session id server-side is a separate design question,
left out of scope here.

tests/ClaudeDo.Ui.Tests/ViewModels/MissionControl/ConPtyPaneViewModelTests.cs

@@ -134,5 +134,21 @@ public class ConPtyPaneViewModelTests
         Assert.True(pane.RetryCommand.CanExecute(null));
     }
 
+    // ── Died-at-startup (fix: an immediate ConPTY exit used to leave StartError null, so Retry
+    //    never became reachable and the pane was stuck reusing the same persisted session id) ──
+
+    [Fact]
+    public void RetryCommand_Enabled_AfterProcessExitsNonzeroRightAfterLaunch()
+    {
+        using var pane = NewTaskPane(NeverCompletes);
+        SetRunning(pane);
+        Assert.False(pane.RetryCommand.CanExecute(null));
+
+        pane.Terminal.OnSessionProcessExited(null, 1);
+
+        Assert.NotNull(pane.Terminal.StartError);
+        Assert.True(pane.RetryCommand.CanExecute(null));
+    }
+
     private static void SetRunning(ConPtyPaneViewModel pane) => pane.Terminal.IsRunning = true;
 }