feat(cli): rewrite input layer with prompt_toolkit and polish UI

- Replaces fragile input() hacks with robust prompt_toolkit.PromptSession
- Native support for multiline paste, history, and clean display
- Restores animated spinner in _thinking_ctx (now safe)
- Replaces boxed Panel with clean header for easier copying
- Adds prompt-toolkit dependency
- Adds new unit tests for input layer

tests/test_cli_input.py

@@ -0,0 +1,58 @@
+import asyncio
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+from prompt_toolkit.formatted_text import HTML
+
+from nanobot.cli import commands
+
+
+@pytest.fixture
+def mock_prompt_session():
+    """Mock the global prompt session."""
+    mock_session = MagicMock()
+    mock_session.prompt_async = AsyncMock()
+    with patch("nanobot.cli.commands._PROMPT_SESSION", mock_session):
+        yield mock_session
+
+
+@pytest.mark.asyncio
+async def test_read_interactive_input_async_returns_input(mock_prompt_session):
+    """Test that _read_interactive_input_async returns the user input from prompt_session."""
+    mock_prompt_session.prompt_async.return_value = "hello world"
+
+    result = await commands._read_interactive_input_async()
+    
+    assert result == "hello world"
+    mock_prompt_session.prompt_async.assert_called_once()
+    args, _ = mock_prompt_session.prompt_async.call_args
+    assert isinstance(args[0], HTML)  # Verify HTML prompt is used
+
+
+@pytest.mark.asyncio
+async def test_read_interactive_input_async_handles_eof(mock_prompt_session):
+    """Test that EOFError converts to KeyboardInterrupt."""
+    mock_prompt_session.prompt_async.side_effect = EOFError()
+
+    with pytest.raises(KeyboardInterrupt):
+        await commands._read_interactive_input_async()
+
+
+def test_init_prompt_session_creates_session():
+    """Test that _init_prompt_session initializes the global session."""
+    # Ensure global is None before test
+    commands._PROMPT_SESSION = None
+    
+    with patch("nanobot.cli.commands.PromptSession") as MockSession, \
+         patch("nanobot.cli.commands.FileHistory") as MockHistory, \
+         patch("pathlib.Path.home") as mock_home:
+        
+        mock_home.return_value = MagicMock()
+        
+        commands._init_prompt_session()
+        
+        assert commands._PROMPT_SESSION is not None
+        MockSession.assert_called_once()
+        _, kwargs = MockSession.call_args
+        assert kwargs["multiline"] is False
+        assert kwargs["enable_open_in_editor"] is False