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]

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

@@ -36,6 +36,7 @@ public class InteractiveTerminalViewModelTests
 
         Assert.True(terminal.HasExited);
         Assert.Null(terminal.StartError);
+        Assert.True(terminal.HasExitedAfterStartup); // no StartError banner, so the host needs this signal instead
     }
 
     [Fact]
@@ -48,6 +49,30 @@ public class InteractiveTerminalViewModelTests
 
         Assert.True(terminal.HasExited);
         Assert.Null(terminal.StartError);
+        Assert.True(terminal.HasExitedAfterStartup);
+    }
+
+    // ── HasExitedAfterStartup (drives the host's "session ended" banner — every death that
+    //    ISN'T already covered by StartError) ─────────────────────────────────────────────────
+
+    [Fact]
+    public void FreshTerminal_HasExitedAfterStartupIsFalse()
+    {
+        using var terminal = new InteractiveTerminalViewModel(() => LaunchedAt);
+
+        Assert.False(terminal.HasExitedAfterStartup);
+    }
+
+    [Fact]
+    public void ProcessExitsNonzero_WithinGraceWindow_HasExitedAfterStartupIsFalse()
+    {
+        using var terminal = new InteractiveTerminalViewModel(() => LaunchedAt);
+        terminal.IsRunning = true;
+
+        terminal.OnSessionProcessExited(null, 1); // startup failure — StartError covers it instead
+
+        Assert.NotNull(terminal.StartError);
+        Assert.False(terminal.HasExitedAfterStartup);
     }
 
     // ── ExitedAfterStartup (a session that ran, then broke -- used to be a silent dead tile;