fix(context): bound native filesystem tool output in Responses mode

Chat-completions mode converts filesystem CustomTools to FunctionTools
(which bounds their result), but the Responses-API path kept them native
and unbounded, so a large read_file could still exhaust the context
window. Always configure the Filesystem capability to head+tail bound
tool output in both modes.

tests/test_agent_factory_shell.py

@@ -3,10 +3,11 @@
 from __future__ import annotations
 
 import json
+from types import SimpleNamespace
 from typing import Any, cast
 
 import pytest
-from agents.tool import FunctionTool
+from agents.tool import CustomTool, FunctionTool
 
 from strix.agents import factory
 from strix.config import load_settings
@@ -77,3 +78,35 @@ async def test_wrap_exec_command_preserves_explicit_shell(shell: str) -> None:
     )
 
     assert json.loads(captured["raw_input"])["shell"] == shell
+
+
+@pytest.mark.asyncio
+async def test_responses_filesystem_custom_tool_output_is_bounded() -> None:
+    # In Responses-API mode filesystem tools stay native CustomTools; a large
+    # read must still be head+tail bounded before it enters history.
+    async def invoke(_ctx: Any, _inp: str) -> str:
+        return "line\n" * 50_000
+
+    toolset = SimpleNamespace(
+        read_file=CustomTool(name="read_file", description="read", on_invoke_tool=invoke)
+    )
+    factory._configure_filesystem_tools(toolset, chat_completions=False)
+
+    assert isinstance(toolset.read_file, CustomTool)
+    result = await toolset.read_file.on_invoke_tool(cast("Any", None), "{}")
+
+    assert "truncated" in result
+    assert len(result) < len("line\n" * 50_000)
+
+
+@pytest.mark.asyncio
+async def test_chat_completions_filesystem_custom_tool_becomes_function_tool() -> None:
+    async def invoke(_ctx: Any, _inp: str) -> str:
+        return "ok"
+
+    toolset = SimpleNamespace(
+        read_file=CustomTool(name="read_file", description="read", on_invoke_tool=invoke)
+    )
+    factory._configure_filesystem_tools(toolset, chat_completions=True)
+
+    assert isinstance(toolset.read_file, FunctionTool)