[FEATURE] Threads: ask follow-up questions under one message without leaving the history

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Long sessions are hard to read and to question.

When I read an old answer and I have a follow-up question, I must scroll to the end of the chat and ask there. Then I must scroll back to the history to continue reading. In a long session this costs much time. It is worst in the morning, when I read the results of a long run and have many questions about earlier answers.

The follow-up answer also lands at the end, far from the message it belongs to. The chat becomes one flat list, and the link between question and answer is lost.

Proposed Solution

Add threads, like in Jira comments.

• On any message or answer, I can open a thread.
• My question and Claude's answer appear UNDER the main message, in a thread view with a tab or an expandable block.
• The view is a tree: main message > sub question > answer.
• One level of thread under each main message. A question about a thread answer becomes a further sub-level.
• The main chat keeps its flow. A thread shows a count (for example "3 replies").
• The session context includes the thread, so Claude knows which message the question is about.

Alternative Solutions

• I scroll to the end and quote the old message by hand. This is slow and loses the link.
• I open a new session. Claude loses the context.
• I use rewind or checkpoints. These change the history, and I only want to ask a question.

Priority

Critical - Blocking my work

Feature Category

Interactive mode (TUI)

Use Case Example

1. Claude works on a long task overnight and writes 20 answers.
2. In the morning I read answer 7 and have a question about it.
3. Today: I scroll to the end, type the question, read the answer, scroll back to answer 7.
4. Proposed: I open a thread on answer 7, ask my question, read the answer under answer 7, and continue with answer 8.

Additional Context

Similar feature: threads in Jira task commants, Slack and Discord, and comment threads in GitHub review.
Environment: VS Code extension, Windows 11. also pls do this for desctop claude code app on windows