Feature request: show context used and remaining per agent

## Feature request

Please show **context used / context remaining** for each agent in the chat UI.

## What I want

In multi-agent / delegated workflows, I want visibility into how much context window each agent is using and how much remains.

Examples of useful information:
- Lead: context used / remaining
- Reviewer: context used / remaining
- Validator: context used / remaining
- Sub-agent X: context used / remaining

Even approximate values are useful if exact numbers are not always available.

## Why this matters

This would help a lot when working with long-running or orchestration-heavy chats, because I want to know:
- which agent is close to context exhaustion
- whether compression/summarization is likely soon
- whether a quiet / degraded response is related to context pressure
- how healthy each agent session is over time

## Suggested UI options

Possible implementations:
- small per-agent context badge
- hover metadata showing used / remaining / total window
- session diagnostics panel for the active conversation
- timeline/analytics view that includes context pressure

## Scope

This request is specifically about **per-agent context visibility**, not only a global total for the whole chat.

No files or tests are named; start by locating the chat UI and the per-agent session data exposed by the VS Code extension. Determine whether context used, remaining, and total are available for each agent, then define the UI surface and verify that every visible agent reports the requested values or an explicit unavailable state.

Written by the indexing model from the issue text.