[feature] 議論の継続機能

## 背景
- 現在ラウンド数を入力して、議論を行なっているが、議論がまとまらない場合や追加で議論を続けさせたい場合は対応できず、最初から議論をやり直す必要があるので、議論の継続機能によるその課題を解決する

No files, tests, or entry points are identified in the issue. First map how the current round count starts and ends a discussion, then clarify how continuation should preserve prior context and accept additional rounds. Done should include an agreed user flow and tests for continuing an unresolved discussion.

Written by the indexing model from the issue text.