Agent session context transfer truncates delegated results without a lossless handoff

## Type of issue

Feature request / data-loss bug in Agents Window session handoff

## Reproduction

1. In VS Code Insiders Agents Window, delegate a long-session research or synthesis task to another Copilot CLI Agent Host chat.
2. After it completes, read that peer chat through session context using digest or full detail.
3. The returned transcript reports `truncated: true` and clips the assistant's single completed response, while `hasMoreHistory` is `false`.
4. There is no message-level continuation cursor or other way to retrieve the omitted remainder.

Observed on 2026-09-17 with current-session peer chats.

## Expected

Completed delegated work can be transferred back to the requesting chat without losing content. This could use message-level pagination/continuation, an automatic artifact or attachment handoff, or another supported full-result channel. If truncation is unavoidable, the response should expose an actionable continuation mechanism.

## Actual

The parent chat receives only the beginning of the completed response. The missing history, decisions, and source links cannot be retrieved from the session-context result.

Asking the delegated agent to restate its findings within a smaller limit discards detail and defeats delegation as a way to isolate research context. The current workaround is to anticipate the limit and ask the agent to write a shared file. That workaround is not obvious, may not work across providers or environments, and can require rerunning the research.

## Environment

- VS Code Insiders
- Agents Window
- Copilot CLI Agent Host
- Peer/current-session chats

Private session IDs and researched content are intentionally omitted.

Start in the VS Code Insiders Agents Window flow for session-context reads, especially digest and full-detail results from current-session peer chats handled by the Copilot CLI Agent Host. Trace how truncation and hasMoreHistory are returned, then verify that a completed delegated response is fully retrievable or exposes an actionable continuation, using the reproduction steps as the acceptance check.

Written by the indexing model from the issue text.