perf(computer_use): cap capture size and cache vision routing

Cut steady-state Computer Use latency without changing default behavior
or waiting on cua-driver:

- Cap screenshots via set_config(max_image_dimension) on session start
  (config: computer_use.max_image_dimension, default 1456)
- Cache aux-vision routing per (provider, model) so captures skip
  repeated load_config()
- Add computer_use.capture_after_mode (default som) so users can opt
  follow-ups down to ax (elements only) for speed

tests/computer_use/test_cua_perf_knobs.py

@@ -0,0 +1,66 @@
+"""Behavior contracts for computer_use latency knobs."""
+
+from unittest.mock import patch
+
+from tools.computer_use import cua_backend
+from tools.computer_use import tool as cu_tool
+
+
+def test_max_image_dimension_default():
+    with patch("hermes_cli.config.load_config", return_value={}):
+        assert cua_backend._computer_use_max_image_dimension() == 1456
+
+
+def test_max_image_dimension_zero_disables():
+    with patch(
+        "hermes_cli.config.load_config",
+        return_value={"computer_use": {"max_image_dimension": 0}},
+    ):
+        assert cua_backend._computer_use_max_image_dimension() is None
+
+
+def test_capture_after_mode_default_som():
+    with patch("hermes_cli.config.load_config", return_value={}):
+        assert cu_tool._capture_after_mode() == "som"
+
+
+def test_capture_after_mode_ax_override():
+    with patch(
+        "hermes_cli.config.load_config",
+        return_value={"computer_use": {"capture_after_mode": "ax"}},
+    ):
+        assert cu_tool._capture_after_mode() == "ax"
+
+
+def test_capture_after_mode_invalid_falls_back_to_som():
+    with patch(
+        "hermes_cli.config.load_config",
+        return_value={"computer_use": {"capture_after_mode": "bogus"}},
+    ):
+        assert cu_tool._capture_after_mode() == "som"
+
+
+def test_aux_vision_route_caches_per_provider_model(monkeypatch):
+    cu_tool._AUX_VISION_ROUTE_CACHE.clear()
+    calls = {"n": 0}
+
+    monkeypatch.setattr(
+        "agent.auxiliary_client._read_main_provider", lambda: "openai"
+    )
+    monkeypatch.setattr(
+        "agent.auxiliary_client._read_main_model", lambda: "gpt-test"
+    )
+
+    def fake_load():
+        calls["n"] += 1
+        return {"auxiliary": {"vision": {}}}
+
+    monkeypatch.setattr("hermes_cli.config.load_config", fake_load)
+    monkeypatch.setattr(
+        "tools.computer_use.vision_routing.should_route_capture_to_aux_vision",
+        lambda *a, **k: True,
+    )
+
+    assert cu_tool._should_route_through_aux_vision() is True
+    assert cu_tool._should_route_through_aux_vision() is True
+    assert calls["n"] == 1