spawn terminal panes in the open workspace root (T-381)

The shell spawned with Directory.current — $HOME for desktop-entry
launches, and stale after a project switch since the process CWD
never moves. Use the kernel project root, falling back to the
process CWD only when no project is open.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/builtin/terminal/src/terminal_pane.dart

@@ -78,7 +78,9 @@ class _TerminalPaneState extends State<TerminalPane> {
     }
 
     final shell = Platform.environment['SHELL'] ?? '/bin/bash';
-    final cwd = Directory.current.path;
+    // The open workspace, not Directory.current — a desktop launch starts
+    // in $HOME and a project switch doesn't move the process CWD (T-381).
+    final cwd = _kernel?.project.current?.path ?? Directory.current.path;
 
     final response = await ipc.request(
       'pane.spawn',