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.

cli-config.yaml.example

@@ -285,7 +285,7 @@ agent:
   # Reasoning effort level (OpenRouter and Nous Portal)
   # Controls how much "thinking" the model does before responding.
   # Options: "xhigh" (max), "high", "medium", "low", "minimal", "none" (disable)
-  reasoning_effort: "xhigh"
+  reasoning_effort: "medium"
   
   # Predefined personalities (use with /personality command)
   personalities: