feat(tui): add average TPS to status bar

[KonstantinMirin]

Issue for this PR

Closes #22344

Type of change

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

What does this PR do?

Adds average output tokens-per-second (TPS) to the TUI status bar, displayed before context size after a response completes. The metric is computed from data already on AssistantMessage: tokens.output / ((time.completed - time.created) / 1000).

Shows up as "42 t/s · 1.2K (12%) · $0.05" in:

• Prompt footer
• Subagent footer
• Sidebar context plugin

TPS only appears on completed messages (requires time.completed), so it won't show mid-stream or for incomplete responses.

How did you verify your code works?

• bun typecheck passes across all packages
• Manually tested in TUI: TPS appears alongside context/cost after a response completes

Screenshots / recordings

N/A — TUI text change

Checklist

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

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found several related PRs that may be duplicates or closely related to PR #22345:

1. PR feat(tui): show tokens per second in the footer and sidebar  #21133 - "feat(tui): show tokens per second in the footer and sidebar"

   • This appears highly related as it also adds TPS display to TUI footer and sidebar

2. PR feat(tui): show tokens per second in assistant message footer #14493 - "feat(tui): show tokens per second in assistant message footer"

   • Similar feature focused on displaying TPS in the assistant message footer

3. PR feat(tui): add tokens per second to response footer #12721 - "feat(tui): add tokens per second to response footer"

   • Related feature adding tokens per second metrics to response footer

4. PR feat(tui): add native status line template system #13885 - "feat(tui): add native status line template system"

   • May be related as it deals with status bar/line customization

These PRs should be reviewed to determine if they're addressing the same feature or if there's overlap in implementation. PR #21133 seems particularly relevant given the matching scope (footer, sidebar, TPS display).

[KonstantinMirin]

Closing in favor of #21133 which already implements TPS display with live streaming support via message.part.delta events, a proper tps.ts utility, and Locale.tokensPerSec. This PR only added post-completion average TPS which is a subset of what #21133 provides.