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.

src/ClaudeDo.Ui/ViewModels/InteractiveTerminalViewModel.cs

@@ -13,9 +13,18 @@ namespace ClaudeDo.Ui.ViewModels;
 /// </summary>
 public sealed partial class InteractiveTerminalViewModel : ViewModelBase, IDisposable
 {
+    // A process that dies within this window of becoming "running" is treated as a startup
+    // failure (see OnSessionProcessExited) rather than a normal session end, even though the
+    // spawn itself succeeded — e.g. `claude --session-id <guid>` exiting immediately on an
+    // auth/network hiccup. Long enough to clear normal CLI startup, short enough not to
+    // mistake a real crash-after-use for a start failure.
+    private static readonly TimeSpan StartupGraceWindow = TimeSpan.FromSeconds(5);
+
     private readonly PtyTerminalSession _session = new();
+    private readonly Func<DateTime> _utcNow;
     private TerminalControl? _control;
     private TerminalLaunchDescriptor? _pendingDescriptor;
+    private DateTime? _startedAtUtc;
 
     [ObservableProperty] private bool _isRunning;
     [ObservableProperty] private bool _hasExited;
@@ -27,12 +36,23 @@ public sealed partial class InteractiveTerminalViewModel : ViewModelBase, IDispo
     /// can show a spinner instead of an empty black pane.</summary>
     public bool IsStarting => !IsRunning && !HasExited && StartError is null;
 
-    partial void OnIsRunningChanged(bool value)   => OnPropertyChanged(nameof(IsStarting));
+    partial void OnIsRunningChanged(bool value)
+    {
+        OnPropertyChanged(nameof(IsStarting));
+        if (value) _startedAtUtc = _utcNow();
+    }
+
     partial void OnHasExitedChanged(bool value)   => OnPropertyChanged(nameof(IsStarting));
     partial void OnStartErrorChanged(string? value) => OnPropertyChanged(nameof(IsStarting));
 
-    public InteractiveTerminalViewModel()
+    public InteractiveTerminalViewModel() : this(() => DateTime.UtcNow)
     {
+    }
+
+    /// <summary>Test-only seam for controlling "time since launch" without real delays.</summary>
+    internal InteractiveTerminalViewModel(Func<DateTime> utcNow)
+    {
+        _utcNow = utcNow;
         _session.ProcessExited += OnSessionProcessExited;
     }
 
@@ -76,11 +96,18 @@ public sealed partial class InteractiveTerminalViewModel : ViewModelBase, IDispo
         }
     }
 
-    private void OnSessionProcessExited(object? sender, int exitCode)
+    /// <summary>Internal (not private) so tests can drive it directly without a real ConPTY spawn.</summary>
+    internal void OnSessionProcessExited(object? sender, int exitCode)
     {
         IsRunning = false;
         HasExited = true;
         ExitCode = exitCode;
+
+        // Died at startup: route through the same banner as a launch-time failure instead of
+        // leaving a dead terminal with no Retry affordance (ConPtyPaneViewModel.CanRetry
+        // requires StartError).
+        if (exitCode != 0 && _startedAtUtc is { } startedAt && _utcNow() - startedAt < StartupGraceWindow)
+            StartError = $"Session exited immediately (code {exitCode}).";
     }
 
     /// <summary>Reports a failure that happened before <see cref="Start"/> could be called (e.g. the