Feature request: preserve ACP chat history across VS Code restarts

## Feature request

Please preserve chat history when VS Code is closed and reopened.

## Problem

Right now, when VS Code closes, the ACP chat history effectively disappears from the UI. That makes long-running workflows much harder to manage.

## Expected behavior

When I reopen VS Code, I want to be able to:
- see prior chat history for the active/recent ACP conversation
- resume context from where I left off
- avoid losing visibility into previous prompts, answers, tool activity, and workflow progress

## Why this matters

For coding agents and multi-step workflows, the chat is part of the working context. Losing the visible history on restart makes it harder to:
- continue interrupted work
- remember what was already asked
- audit previous agent actions
- trust that the session is resumable

## Suggested behavior

Possible approaches:
- restore the most recent conversation automatically on reopen
- keep recent conversations in a session list
- persist chat transcript/UI state and reload it when the extension activates again

## Scope

This request is specifically about **visible ACP chat history persistence across VS Code restarts**, not just reconnecting to an agent process.

No files or tests are named in the issue. Start by locating the extension activation path and the ACP chat UI/state lifecycle, then determine where conversation history can be persisted and restored. Done means visible recent or active chat history survives a VS Code restart and can be resumed.

Written by the indexing model from the issue text.