Feature request: show timestamps and per-agent token usage in the chat UI

## Feature request

Please add better visibility for **timestamps** and **per-agent token usage** in the ACP chat UI.

## What I want to see

### 1. Timestamps
For chat activity, I want timestamps visible so I can tell:
- when a message arrived
- when a tool started/finished
- when an agent handoff happened
- how long the system was quiet between updates

This is especially useful when diagnosing whether an agent is still working, stalled, or has gone quiet too early.

### 2. Token usage per agent
In multi-agent / delegated workflows, I want to see token usage broken down by agent or role.

Examples:
- Lead: input/output tokens
- Reviewer: input/output tokens
- Validator: input/output tokens
- Sub-agent X: input/output tokens

Even a lightweight per-agent breakdown would be very helpful for understanding:
- which agent is doing the heavy lifting
- where cost is going
- whether delegation is actually worth it

## Why this matters

For orchestration-heavy workflows, the current UI can feel too opaque. Timestamps and token usage make it easier to:
- debug workflow silence or latency
- understand handoffs between agents
- estimate cost/efficiency by role
- trust what the system is doing over time

## Suggested UI options

Possible implementations could include:
- timestamps next to messages / tool events / handoff events
- hover or expandable metadata per message/turn
- per-agent token badges or a summary panel
- a compact analytics/timeline view for the active session

## Scope

This request is specifically about **visibility in the ACP Client UI** for:
- event timing
- per-agent token usage

not just aggregate session totals.

Start by locating the ACP Client UI entry points and the event or session data available for chat activity. Check whether timestamps, handoffs, tool timing, and per-agent input/output usage are exposed by the current flow. Done means the UI visibly presents event timing and agent-level token usage rather than only aggregate session totals.

Written by the indexing model from the issue text.