QA: Refactor similarity calculation and improve state management logic

src/heart_flow/sub_mind.py

@@ -78,7 +78,7 @@ def calculate_replacement_probability(similarity: float) -> float:
         # p = 3.5 * s - 1.4
         probability = 3.5 * similarity - 1.4
         return max(0.0, probability)
-    elif 0.6 < similarity < 0.9:
+    else: # 0.6 < similarity < 0.9
         # p = s + 0.1
         probability = similarity + 0.1
         return min(1.0, max(0.0, probability))
@@ -169,7 +169,7 @@ class SubMind:
 
         last_cycle = history_cycle[-1] if history_cycle else None
         # 上一次决策信息
-        if last_cycle != None:
+        if last_cycle is not None:
             last_action = last_cycle.action_type
             last_reasoning = last_cycle.reasoning
             is_replan = last_cycle.replanned