fix(exec): bypass cmd.exe for multi-line python -c commands on Windows

On Windows, cmd.exe /c treats newlines as command separators, silently
dropping code after the first line in `python -c "..."` commands. This
causes multi-line inline Python to produce no output with exit code 0.

Detect multi-line `python -c` commands on Windows, parse them into exec
args via `_split_python_c_args`, and use `create_subprocess_exec` to
bypass cmd.exe entirely. Same principle as Codex's Rust `Command::args()`.

Applied to both the direct execution path and the session spawn path.
Added unit tests for the parser and the exec-vs-shell branching logic.

tests/tools/test_exec_platform.py

@@ -116,7 +116,7 @@ class TestSpawnUnix:
 class TestSpawnWindows:
 
     @pytest.mark.asyncio
-    async def test_uses_create_subprocess_shell(self):
+    async def test_single_line_uses_shell(self):
         env = {"COMSPEC": r"C:\Windows\system32\cmd.exe", "PATH": ""}
         with (
             patch("nanobot.agent.tools.shell._IS_WINDOWS", True),
@@ -132,7 +132,7 @@ class TestSpawnWindows:
         assert kwargs["stdin"] == asyncio.subprocess.DEVNULL
 
     @pytest.mark.asyncio
-    async def test_passes_cwd_and_env(self):
+    async def test_single_line_passes_cwd_and_env(self):
         env = {"PATH": "/usr/bin"}
         with (
             patch("nanobot.agent.tools.shell._IS_WINDOWS", True),
@@ -145,6 +145,27 @@ class TestSpawnWindows:
         assert kwargs["cwd"] == r"C:\work"
         assert kwargs["env"] == env
 
+    @pytest.mark.asyncio
+    async def test_multiline_uses_powershell(self):
+        env = {"PATH": ""}
+        with (
+            patch("nanobot.agent.tools.shell._IS_WINDOWS", True),
+            patch("asyncio.create_subprocess_exec", new_callable=AsyncMock) as mock_exec,
+        ):
+            mock_exec.return_value = AsyncMock()
+            await ExecTool._spawn('python -c "print(1)\nprint(2)"', r"C:\work", env)
+
+        args = mock_exec.call_args[0]
+        assert args[0] == "powershell"
+        assert "-NoProfile" in args
+        assert "-Command" in args
+        assert "print(1)" in args[-1]
+        assert "print(2)" in args[-1]
+
+        kwargs = mock_exec.call_args[1]
+        assert kwargs["cwd"] == r"C:\work"
+        assert kwargs["env"] == env
+
 
 # ---------------------------------------------------------------------------
 # path_append
@@ -352,3 +373,85 @@ class TestExtractAbsolutePaths:
         cmd = "echo hello"
         paths = ExecTool._extract_absolute_paths(cmd)
         assert paths == []
+
+
+# ---------------------------------------------------------------------------
+# Windows multi-line command PowerShell fallback
+# ---------------------------------------------------------------------------
+
+class TestWindowsMultilineExec:
+    """Verify multi-line commands on Windows route through PowerShell."""
+
+    @pytest.mark.asyncio
+    async def test_multiline_python_uses_powershell(self):
+        mock_proc = AsyncMock()
+        mock_proc.communicate.return_value = (b"1\n2\n", b"")
+        mock_proc.returncode = 0
+
+        with (
+            patch("nanobot.agent.tools.shell._IS_WINDOWS", True),
+            patch("asyncio.create_subprocess_exec", new_callable=AsyncMock) as mock_exec,
+            patch.object(ExecTool, "_guard_command", return_value=None),
+        ):
+            mock_exec.return_value = mock_proc
+            tool = ExecTool()
+            result = await tool.execute(command='python -c "print(1)\nprint(2)"')
+
+        assert "1" in result
+        assert "2" in result
+        assert "Exit code: 0" in result
+        args = mock_exec.call_args[0]
+        assert args[0] == "powershell"
+
+    @pytest.mark.asyncio
+    async def test_multiline_node_uses_powershell(self):
+        mock_proc = AsyncMock()
+        mock_proc.communicate.return_value = (b"1\n", b"")
+        mock_proc.returncode = 0
+
+        with (
+            patch("nanobot.agent.tools.shell._IS_WINDOWS", True),
+            patch("asyncio.create_subprocess_exec", new_callable=AsyncMock) as mock_exec,
+            patch.object(ExecTool, "_guard_command", return_value=None),
+        ):
+            mock_exec.return_value = mock_proc
+            tool = ExecTool()
+            result = await tool.execute(command='node -e "console.log(1)\nconsole.log(2)"')
+
+        assert "1" in result
+        args = mock_exec.call_args[0]
+        assert args[0] == "powershell"
+
+    @pytest.mark.asyncio
+    async def test_single_line_uses_shell(self):
+        mock_proc = AsyncMock()
+        mock_proc.communicate.return_value = (b"1\n", b"")
+        mock_proc.returncode = 0
+
+        with (
+            patch("nanobot.agent.tools.shell._IS_WINDOWS", True),
+            patch.object(ExecTool, "_spawn", return_value=mock_proc) as mock_spawn,
+            patch.object(ExecTool, "_guard_command", return_value=None),
+        ):
+            tool = ExecTool()
+            result = await tool.execute(command='python -c "print(1)"')
+
+        assert "1" in result
+        mock_spawn.assert_called_once()
+
+    @pytest.mark.asyncio
+    async def test_unix_unchanged(self):
+        mock_proc = AsyncMock()
+        mock_proc.communicate.return_value = (b"1\n2\n", b"")
+        mock_proc.returncode = 0
+
+        with (
+            patch("nanobot.agent.tools.shell._IS_WINDOWS", False),
+            patch.object(ExecTool, "_spawn", return_value=mock_proc) as mock_spawn,
+            patch.object(ExecTool, "_guard_command", return_value=None),
+        ):
+            tool = ExecTool()
+            result = await tool.execute(command='python -c "print(1)\nprint(2)"')
+
+        assert "1" in result
+        mock_spawn.assert_called_once()