fix(tools): resolve the sandbox interpreter for workspace file scripts

The workspace file tools (read/write/edit/glob/grep) ran a script through
`python`, which a sandbox built from a host rootfs may not provide: those hosts
ship `python3` only, and a read-only /usr prevents creating a `python` shim
inside the jail. Every file tool then failed with `/bin/sh: 1: python: not
found`. Resolve the interpreter with `command -v python3 || command -v python`,
matching the interpreter the Box service already uses for its own scripts.

src/langbot/pkg/provider/tools/loaders/native.py

@@ -754,9 +754,16 @@ class NativeToolLoader(loader.ToolLoader):
         return f'{str(base).rstrip("/")}/{relative}'
 
     async def _run_workspace_file_script(self, script: str, query: pipeline_query.Query) -> dict:
+        # Sandbox images built from a host rootfs may only ship `python3`, and a
+        # read-only /usr prevents creating a `python` shim inside the jail, so
+        # resolve the interpreter instead of assuming its name.
+        command = (
+            'PYTHON_BIN=$(command -v python3 || command -v python); '
+            f"\"$PYTHON_BIN\" - <<'PY'\n{script}\nPY"
+        )
         result = await self.ap.box_service.execute_tool(
             {
-                'command': f"python - <<'PY'\n{script}\nPY",
+                'command': command,
                 'timeout_sec': 30,
             },
             query,

tests/unit_tests/provider/test_tool_manager_native.py

@@ -851,3 +851,35 @@ async def test_box_grep_script_serializes_optional_include_as_python(monkeypatch
             values[statement.targets[0].id] = ast.literal_eval(statement.value)
     assert values['include'] == include
     assert values['path'] == '/workspace'
+
+
+@pytest.mark.asyncio
+async def test_workspace_file_script_resolves_the_sandbox_interpreter():
+    """Sandboxes built from a host rootfs may only ship `python3`."""
+
+    captured: dict = {}
+
+    async def execute_tool(payload, query):
+        captured.update(payload)
+        return {'ok': True, 'stdout': '{"ok": true}'}
+
+    box_service = SimpleNamespace(
+        available=True,
+        execute_tool=execute_tool,
+        require_workspace_sandbox=AsyncMock(),
+    )
+    loader = NativeToolLoader(SimpleNamespace(box_service=box_service, logger=Mock()))
+    query = SimpleNamespace(
+        _execution_context=_CONTEXT,
+        bot_uuid=None,
+        pipeline_uuid=None,
+        query_uuid=None,
+    )
+    query._box_binding = RunBoxBinding('run', 'box', {}, 'run')
+
+    result = await loader._run_workspace_file_script('print("x")', query)
+
+    assert result == {'ok': True}
+    command = captured['command']
+    assert command.startswith('PYTHON_BIN=$(command -v python3 || command -v python)')
+    assert command.endswith(" - <<'PY'\nprint(\"x\")\nPY")