Add a context % output on the main/general output area

### Describe the feature or problem you'd like to solve

I don't want to have to `/context` every few minutes to see how my model's context is filling up. Optimal fix is you have it be super smart about when to compact so I don't have to track it. But until then, give me a way to see context easily so I can track it and make a good call on when to /compact

### Proposed solution

Next to the current model name and cost seems like a good place to put context %. You can do it as some kind of iconography like VS Code does with the pie chart showing context fill percentage, or you could just have `context: 58%` and optionally color on a gradient from green to red as it fills up.

### Example prompts or workflows

I'm not sure what to put here other than:
- I'm working along and just before I start another demanding ask related to the last one, I notice some red text on the screen that indicates I'm at 90% of available context. I chose to compact at this point rather than partway through the next task.

### Additional context

Here's a mockup showing where I think it could go (in red rectangle):

Makes sense to go with the model name as context *is* model dependent.