feat(cli): display model reasoning content during streaming

Add show_reasoning config (default: False) to display model
thinking/reasoning content in the TUI during streaming.  Reasoning
is emitted via a new emit_reasoning hook on AgentHook, gated by the
channels config.  Display uses ✻ prefix with dim italic styling.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

nanobot/agent/hook.py

@@ -48,6 +48,9 @@ class AgentHook:
     async def before_execute_tools(self, context: AgentHookContext) -> None:
         pass
 
+    async def emit_reasoning(self, reasoning_content: str | None) -> None:
+        pass
+
     async def after_iteration(self, context: AgentHookContext) -> None:
         pass
 
@@ -95,6 +98,9 @@ class CompositeHook(AgentHook):
     async def before_execute_tools(self, context: AgentHookContext) -> None:
         await self._for_each_hook_safe("before_execute_tools", context)
 
+    async def emit_reasoning(self, reasoning_content: str | None) -> None:
+        await self._for_each_hook_safe("emit_reasoning", reasoning_content)
+
     async def after_iteration(self, context: AgentHookContext) -> None:
         await self._for_each_hook_safe("after_iteration", context)