fix(core/agent): 移除 max_steps 默认值

设计要求调用方显式传入 max_steps（P4: 显式优于隐式）。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/agent/loop.py

@@ -72,7 +72,7 @@ class AgentLoop:
     def __init__(
         self,
         llm: LLMProvider,
-        max_steps: int = 15,
+        max_steps: int,
         max_retries: int = 3,
     ) -> None:
         self._llm = llm