Add /context command to display session token usage

[bpeers01]

Problem
OpenCode already tracks per-request token usage, but there’s no quick way to view it in the TUI. This makes it hard to check context headroom without leaving the session.

Proposal
Add a session-level /context command that opens a dialog showing last-request token usage, context limit, usable input budget, and cache hit ratio.

Scope
TUI only. No new tracking data; just a UI for existing usage fields.

Why this matters
Improves visibility into prompt headroom and cache usage while staying in the session flow.

Verification
I will include a short repro in the PR and before/after screenshots for the dialog.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Session context usage (similar to /context in Claude) #6152: [FEATURE]: Session context usage (similar to /context in Claude) - Requests a TUI dialog showing context window breakdown and token usage

Feel free to ignore if this doesn't address your specific case.