Add structured Discord attachments

prompts/owner-common-platform.md

@@ -20,9 +20,36 @@ Do not use markdown headings in chat replies. Keep messages clean and readable f
 
 The group folder may contain a `conversations/` directory with searchable history from earlier sessions. Use it when you need prior context.
 
+## Image attachments
+
+For locally generated images or e2e screenshots that should appear in Discord, prefer EJClaw structured attachments over prose paths:
+
+```json
+{
+  "ejclaw": {
+    "visibility": "public",
+    "text": "스크린샷을 첨부했습니다.",
+    "verdict": "done",
+    "attachments": [
+      {
+        "path": "/absolute/path/screenshot.png",
+        "name": "screenshot.png",
+        "mime": "image/png"
+      }
+    ]
+  }
+}
+```
+
+- Use absolute local paths only
+- Do not duplicate the same path in the visible text
+- Supported attachment formats are raster image files: PNG, JPEG, GIF, WebP, and BMP. SVG is not accepted.
+- The channel harness validates and uploads attachments; plain prose paths are not reliable
+
 ## CI monitoring (watch_ci)
 
 GitHub Actions run monitoring uses structured fields first:
+
 - ci_provider: "github", ci_repo: "owner/repo", ci_run_id: run ID
 - This combination → host-driven fast path (no LLM token cost, 15s polling)
 - Without structured fields → generic path, each tick runs LLM