Context usage indicator in the chat window

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### 1. Is this request related to a challenge you're experiencing? Tell me about your story.

I am building a lot of agents with the new Agent v2, especially the interaction with workflows and custom tools.
Really loving it so far.

The chat window is my main entry point for talking to access-scoped agents with specialized tools. Automations run with the same agent but talking through the chat window lets useres do one-off requests and analysis for example.

The problem is that I have no idea how full the context is. A conversation gets long, or the agent reads a big file with a tool, and the context fills up without me seeing any of it. I am just guessing.

That matters for two reasons.
- Cost, because the history gets sent along on every turn.
- Quality, because the closer a model gets to its limit, the dumber it gets.

Right now the only way to check is to open Logs & Ann. and read the token count per message, which is not something you do in the middle of a conversation.

So currently its hard to judge if i should start a new conversation or not.

### 2. Additional context or comments

Ideal would be a something in the chat window that shows how much of the model's context the current conversation is using.
A percentage or a small bar, with the token count on hover.

The data is already there. Every message stores its prompt tokens, and the model's context size is known from the model config.

It would help most in agent apps, where tool outputs are part of the history too, so the context grows a lot faster than you would guess from just looking at the messages on screen.

Thanks for building this awesome tool! Really love the direction its going!

### 3. Can you help us with this feature?

- [ ] I am interested in contributing to this feature.

Start at the chat window and compare its current conversation data with the token counts shown in Logs & Ann.; review how the model configuration exposes its context size. The feature is done when the chat displays current context usage as a percentage or bar and exposes the token count on hover, including tool outputs in agent conversations.

Written by the indexing model from the issue text.