fix(ui): show a banner when a ConPTY session dies after startup

InteractiveTerminalViewModel.HasExitedAfterStartup now covers every
post-startup death that ExitedAfterStartup alone missed (crash, clean
exit, quiet exit inside the grace window) -- previously the tile just
stayed looking alive/thinking with zero signal. ConPtyPaneView binds a
new banner to it (message + Retry + Close); CanRetry simplifies to
"any HasExited state" since the same descriptor factory can relaunch
either kind of death. HandlerRestartCoordinator's worker-side
auto-restart path is untouched.

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

@@ -151,6 +151,30 @@ public class ConPtyPaneViewModelTests
         Assert.True(pane.RetryCommand.CanExecute(null));
     }
 
+    // ── HasExitedAfterStartup banner (fix: a mid-run death used to leave a silent dead tile — no
+    //    StartError, no signal at all) ─────────────────────────────────────────────────────────
+
+    [Fact]
+    public void RetryCommand_Enabled_AfterDeathAfterStartup()
+    {
+        using var pane = NewTaskPane(NeverCompletes);
+
+        DieAfterRunning(pane, exitCode: 1);
+
+        Assert.True(pane.Terminal.HasExitedAfterStartup);
+        Assert.True(pane.RetryCommand.CanExecute(null));
+    }
+
+    [Fact]
+    public void ExitedBannerText_IncludesExitCode()
+    {
+        using var pane = NewTaskPane(NeverCompletes);
+
+        DieAfterRunning(pane, exitCode: 42);
+
+        Assert.Contains("42", pane.ExitedBannerText);
+    }
+
     // ── SessionDiedAfterStartup (feeds the worker's usage-limit auto-restart) ────────────────────
 
     [Fact]