Discord plugin delivers duplicate messages (channel tag + system-reminder)

Bug Description

When using the Discord channel plugin (plugin:discord:discord), every incoming message is delivered twice to the conversation context:

1. As a <channel source="plugin:discord:discord" ...> tag (normal delivery)
2. As a <system-reminder> containing the same <channel> tag with the note "A message arrived from plugin:discord:discord while you were working"

This happens regardless of whether any background tasks are running. Even when the assistant is idle and waiting for input, both deliveries occur.

Expected Behavior

• Messages should be delivered once
• The system-reminder re-delivery should only occur if the assistant was genuinely busy (e.g., executing a tool or background task) when the message arrived

Actual Behavior

Every Discord message appears twice in the conversation context. The assistant must manually deduplicate by responding "Already responded above" to the system-reminder copy.

Impact

• Wastes context window (each message consumes ~2x the tokens)
• Requires the assistant to handle deduplication logic
• Confusing when reviewing the CLI conversation log

Environment

• Claude Code CLI (latest)
• Discord channel plugin (plugin:discord:discord)
• Platform: Linux (WSL2)