feat: dynamic tabbed workspace with multi-file and multi-terminal support

Replaces the fixed 3-tab workspace (Editor/Diff/Terminal) with a VS Code-style
dynamic tab system. Users can now open multiple files, terminals, and diffs in
closeable tabs with type icons, modified indicators, and file deduplication.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/test_widgets.py

@@ -383,8 +383,7 @@ class TestTerminalPane:
 
         pane = TerminalPane(cwd=tmp_path)
         assert pane.cwd == tmp_path
-        assert pane._history == []
-        assert pane._history_index == 0
+        assert pane._terminal is None
 
     def test_default_cwd(self):
         """Test default cwd is current directory."""
@@ -401,13 +400,6 @@ class TestTerminalPane:
         pane._cwd = tmp_path
         assert pane.cwd == tmp_path
 
-    def test_command_submitted_message(self):
-        """Test CommandSubmitted message."""
-        from clide.widgets.components.terminal_pane import TerminalPane
-
-        msg = TerminalPane.CommandSubmitted("ls -la")
-        assert msg.command == "ls -la"
-
 
 class TestProblemsView:
     """Tests for ProblemsView component."""