feat(tui): add /usage command for token and cost usage

[anxkhn]

Issue for this PR

Related to #9281

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a /usage command (alias /cost) to the TUI. It pulls the session list from the SDK client, sums cost and tokens into Today / This Week / This Month / All Time buckets plus a per-model breakdown, and shows them in a dialog. The numbers are the same ones opencode stats already computes, just surfaced in the TUI. It is one new dialog component plus one command registration in app.tsx, all inside the TUI package.

I first wrote this as a plugin, then read the codebase and it looked small enough to do natively. Please treat it as a proposal: happy to change scope or approach, or fold it into the discussion in #9281.

Known limitation: the client session.list is scoped to the current project, whereas opencode stats reads every session from the DB. I can wire it to a server side aggregation if you would prefer all projects.

How did you verify your code works?

• Ran it in the TUI; /usage opens the dialog and renders correctly (screenshot below).
• bun install, then bun run typecheck in packages/tui: passes, no errors.
• oxlint on the new file: clean. app.tsx shows only pre-existing warnings (confirmed unchanged via git stash).
• prettier applied to both changed files; pre-push checks passed.

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[anxkhn]

attaching screenshot.

[github-actions]

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