refactor: 彻底消除硬编码 qq — 平台标识统一为 obv11

- PlatformName() 返回 "obv11"
- platformFormats key: "obv11"
- 所有 platform == "qq" 检查 → "obv11"
- session prefix: platform_qq_ → platform_obv11_
- env var PLATFORM_CHANNELS: obv11:group:xxx
- ethend 平台标识同步更新
- 注释和测试用例同步

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

.env.example

@@ -37,7 +37,7 @@ PROACTIVE_MSG_MIN_GAP_SEC=1800
 
 # ========== 平台接入 ==========
 # --- 平台观察 ---
-PLATFORM_CHANNELS=qq:group:群号
+PLATFORM_CHANNELS=obv11:group:群号
 PLATFORM_THINK_INTERVAL_SEC=600
 # --- 平台桥接 ---
 PLATFORM_BRIDGE_URL=http://localhost:8095