feat: default reasoning effort from xhigh to medium

Reduces token usage and latency for most tasks by defaulting to
medium reasoning effort instead of xhigh. Users can still override
via config or CLI flag. Updates code, tests, example config, and docs.

tests/test_run_agent.py

@@ -498,12 +498,12 @@ class TestBuildApiKwargs:
         assert kwargs["extra_body"]["provider"]["only"] == ["Anthropic"]
 
     def test_reasoning_config_default_openrouter(self, agent):
-        """Default reasoning config for OpenRouter should be xhigh."""
+        """Default reasoning config for OpenRouter should be medium."""
         messages = [{"role": "user", "content": "hi"}]
         kwargs = agent._build_api_kwargs(messages)
         reasoning = kwargs["extra_body"]["reasoning"]
         assert reasoning["enabled"] is True
-        assert reasoning["effort"] == "xhigh"
+        assert reasoning["effort"] == "medium"
 
     def test_reasoning_config_custom(self, agent):
         agent.reasoning_config = {"enabled": False}