docs(schema,config): clarify reasoning_effort semantics for MiMo thinking mode

- Update AgentDefaults.reasoning_effort comment to document "none"
  (disable) and None (preserve provider default).
- Add configuration.md tip explaining MiMo thinking mode behavior.

nanobot/config/schema.py

@@ -88,7 +88,7 @@ class AgentDefaults(Base):
         validation_alias=AliasChoices("toolHintMaxLength"),
         serialization_alias="toolHintMaxLength",
     )  # Max characters for tool hint display (e.g. "$ cd …/project && npm test")
-    reasoning_effort: str | None = None  # low / medium / high / adaptive - enables LLM thinking mode
+    reasoning_effort: str | None = None  # low / medium / high / adaptive / none — LLM thinking effort; None preserves the provider default
     timezone: str = "UTC"  # IANA timezone, e.g. "Asia/Shanghai", "America/New_York"
     bot_name: str = "nanobot"  # Display name shown in CLI prompts (e.g. "{name} is thinking...")
     bot_icon: str = "🐈"  # Short icon (emoji or text) shown next to the bot name in CLI; "" to omit