fix: prevent duplicate Discord messages for long content

[renezander030]

Summary

• During edit-streaming, the >1900 char code path split content into chunks and called channel.say() for each overflow chunk, creating new Discord messages
• The final edit after streaming also split and called channel.say() for overflow chunks -- both paths ran independently, producing duplicate messages
• Fix: during streaming, truncate content over 1900 chars with an ellipsis and only edit the single placeholder message. The final edit handles proper multi-message splitting after streaming completes.

Test plan

• Trigger an agent reply exceeding 1900 chars (e.g., multi-tool-call response with long analysis)
• Verify only one set of chunked messages appears in Discord (no duplicates)
• Verify the streaming preview shows truncated content with ellipsis for long responses
• Verify the final output is correctly split across multiple messages when needed

Fixes #81

[masami-agent]

Hi @renezander030, this PR overlaps with #135 which addresses the same duplicate message issue and has already been reviewed and approved. Would you be okay closing this one in favor of #135? Thanks 🙏

[renezander030]

@masami-agent OK with me. Thanks