Show used tokens inline an not only after closing copilot

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

With the upcomming https://github.blog/news-insights/company-news/github-copilot-is-moving-to-usage-based-billing/ it becomes important to see what consumes tokens (and how much).

### Proposed solution

When I close copilot it shows for example

> Tokens ↑ 275.8k • ↓ 3.5k • 219.4k (cached)

but during a session there is no way to know how many was consumed, neither `/session show`, the status-bar or the summary includes any direct indication.

It would be good to show token information in the status bar, and the session should show the complete summary of *all* interactions for this session - currently the summary at the end resets after resuming a session so there is no way to know how expensive a given session over a long run was.

At best there would be even a `/session stats` that shows what action has taken what tokes, e.g. MCP server interactions, file changes, "thinking" and so on.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start by tracing the status-bar, `/session show`, and session-summary entry points mentioned in the issue. Done means token usage is visible during a session and the session summary remains cumulative after resuming; any per-action statistics would need an agreed scope.

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