feat(interactive): embedded ConPTY terminal host in UI

Adds a self-contained terminal host: PtyTerminalSession drives a Porta.Pty child
directly (custom env) and pumps it through Iciclecreek's XTerm.NET renderer via
TerminalControl.Terminal, bypassing LaunchProcess() so a fully-populated
environment can be passed. InteractiveTerminalView/ViewModel host it with an
order-independent attach/start. Adds Iciclecreek.Avalonia.Terminal 2.0.3.
Sets Process="" to suppress the control's auto-launch of a stray shell.

src/ClaudeDo.Ui/ViewModels/InteractiveTerminalViewModel.cs

@@ -0,0 +1,84 @@
+using System;
+using System.Threading.Tasks;
+using CommunityToolkit.Mvvm.ComponentModel;
+using ClaudeDo.Ui.Services;
+using Iciclecreek.Terminal;
+
+namespace ClaudeDo.Ui.ViewModels;
+
+/// <summary>
+/// Hosts a <see cref="PtyTerminalSession"/> for an embedded ConPTY terminal. The view attaches
+/// its <see cref="TerminalControl"/> once loaded via <see cref="AttachControl"/>; <see cref="Start"/>
+/// can be called before or after attach — whichever happens second triggers the launch.
+/// </summary>
+public sealed partial class InteractiveTerminalViewModel : ViewModelBase, IDisposable
+{
+    private readonly PtyTerminalSession _session = new();
+    private TerminalControl? _control;
+    private TerminalLaunchDescriptor? _pendingDescriptor;
+
+    [ObservableProperty] private bool _isRunning;
+    [ObservableProperty] private bool _hasExited;
+    [ObservableProperty] private int? _exitCode;
+    [ObservableProperty] private string? _startError;
+
+    public InteractiveTerminalViewModel()
+    {
+        _session.ProcessExited += OnSessionProcessExited;
+    }
+
+    /// <summary>Called by the view once its <see cref="TerminalControl"/> has loaded (template applied).</summary>
+    public void AttachControl(TerminalControl control)
+    {
+        _control = control;
+        if (_pendingDescriptor is { } descriptor)
+        {
+            _pendingDescriptor = null;
+            _ = StartCoreAsync(descriptor);
+        }
+    }
+
+    public void Start(TerminalLaunchDescriptor descriptor)
+    {
+        if (_control is null)
+        {
+            _pendingDescriptor = descriptor;
+            return;
+        }
+        _ = StartCoreAsync(descriptor);
+    }
+
+    private async Task StartCoreAsync(TerminalLaunchDescriptor descriptor)
+    {
+        if (_control is null) return;
+        try
+        {
+            StartError = null;
+            await _session.StartAsync(descriptor, _control);
+            IsRunning = true;
+            HasExited = false;
+            ExitCode = null;
+        }
+        catch (Exception ex)
+        {
+            IsRunning = false;
+            HasExited = true;
+            StartError = ex.Message;
+        }
+    }
+
+    private void OnSessionProcessExited(object? sender, int exitCode)
+    {
+        IsRunning = false;
+        HasExited = true;
+        ExitCode = exitCode;
+    }
+
+    public void Kill() => _session.Kill();
+
+    public void Dispose()
+    {
+        _session.ProcessExited -= OnSessionProcessExited;
+        _session.Dispose();
+    }
+}