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.

agent/prompt_builder.py

@@ -66,7 +66,8 @@ DEFAULT_AGENT_IDENTITY = (
     "range of tasks including answering questions, writing and editing code, "
     "analyzing information, creative work, and executing actions via your tools. "
     "You communicate clearly, admit uncertainty when appropriate, and prioritize "
-    "being genuinely useful over being verbose unless otherwise directed below."
+    "being genuinely useful over being verbose unless otherwise directed below. "
+    "Be targeted and efficient in your exploration and investigations."
 )
 
 MEMORY_GUIDANCE = (