Feature request: show when a delegated agent is used and display its name

## Feature request

Make it clearer in the chat UI when a delegated/sub-agent is being used, and show the agent name explicitly.

## Problem

In multi-agent workflows, the user can lose track of:
- whether the main agent is responding directly
- whether a specialist/sub-agent has been invoked
- which agent is currently doing work

That makes the workflow feel opaque, especially for orchestration-heavy setups.

## Requested behavior

When an agent/sub-agent/delegated worker is involved, the UI should show:
- that an agent handoff/delegation happened
- the agent name or role
- optionally whether it is active / completed / returned results

Examples of useful UI patterns:
- inline badge: `Lead`, `Reviewer`, `Validator`, `Subagent`
- timeline row: `Delegated to `
- section header showing which agent produced the current output

## Why this matters

For users running structured workflows, transparency about agent identity makes it much easier to:
- trust what is happening
- debug quiet/stalled sessions
- understand which role produced which result

The issue names no files or tests; start by locating the chat UI and the handling of delegated or sub-agent work in this VS Code extension. Confirm how agent identity and handoff state are represented, then define a UI treatment that shows the delegated agent name and, where available, its active, completed, or returned-results state.

Written by the indexing model from the issue text.