Feature request: clear or collapse completed task lists instead of accumulating them

## Feature request

When the task list/checklist for a chat flow has been completed, it should be cleared (or archived/collapsed) instead of continuing to grow throughout the rest of the conversation.

## Problem

In long-running agent chats, completed task lists can remain visible and keep accumulating even after the work is done. Over time this creates unnecessary UI clutter and makes the active state of the conversation harder to read.

## Expected behavior

Once a task list is fully completed, the UI should do one of the following:
- clear it from the active task area
- collapse it automatically
- move it into a completed/archive section

The important part is that the active task UI should not keep growing forever with completed items.

## Why this matters

This would improve readability during long workflows by making it easier to see:
- what is still active now
- whether a new task list has started
- whether the agent is currently working vs just showing historical completed items

## Suggested behavior

Possible implementations:
- auto-clear when all tasks are marked completed
- auto-collapse completed lists behind a “show completed tasks” toggle
- keep only the current active task list visible by default

## Scope

This request is about the **active task list UI behavior** during a conversation, especially for long-running workflows where multiple completed lists may otherwise accumulate.

No files or tests are named. Start by locating the active task list UI and its completion-state handling in the VS Code extension, then review how completed lists are rendered during long conversations. Done means completed lists no longer accumulate in the active task area, with the chosen clear, collapse, or archive behavior covered by relevant tests.

Written by the indexing model from the issue text.