Add token count option for the new /statusline context window item

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

/statusline was added in 1.0.30, and it lets us show the context window in the status bar. Right now, that item appears to show percentage only.

### Proposed solution

Add a built-in option for the context window item to show token count too, for example 52k/128k, or support modes like percentage | tokens | both.

statusLine.command already exists, but that is a custom-script workaround. This request is for a built-in option in the new /statusline feature.

### Example prompts or workflows

I want /statusline to support a context window display that shows a concrete token value instead of only a percentage.

### Additional context

Related:

- #2052 persistent token/context usage indicator
- #2342 customizable statusline / context % and usage