docs: replace discord screenshots with English/redacted versions

docs/USAGE_GUIDE.en.md

@@ -185,7 +185,7 @@ OCM also supports Discord:
 
 Go to **Agents** → `+ Add Agent` → set **Channel = Discord**:
 
-![](redacted-screenshots/agents-discord-add-agent.jpg)
+![](redacted-screenshots/agents-discord-add-agent.png)
 
 Steps:
 1) Create/select a dedicated Discord **channel** (private recommended)

docs/USAGE_GUIDE.zh-CN.md

@@ -168,7 +168,7 @@ OCM 也支持 Discord：
 
 进入 **Agents** → `+ Add Agent` → 选择 **渠道 = Discord**：
 
-![](redacted-screenshots/agents-discord-add-agent.jpg)
+![](redacted-screenshots/agents-discord-add-agent.png)
 
 步骤：
 1) 创建/选择一个专用 Discord Channel（建议私密）