feat(tui): show usage cache stats

[tt-a1i]

Related Issue

Resolve #884

Problem

/usage already counted prompt-cache tokens as part of input, but it did not show cache read/write counts separately.

What changed

• show per-model prompt cache read/write token counts when they are non-zero
• show aggregate cache read/write counts in the multi-model total section
• keep the existing input/output/total lines unchanged
• add coverage for cache rows, multi-model totals, and no-cache output

Validation

• pnpm --filter @moonshot-ai/kimi-code exec vitest run test/tui/components/messages/usage-panel.test.ts
• pnpm --filter @moonshot-ai/kimi-code run typecheck
• pnpm exec oxlint --type-aware apps/kimi-code/src/tui/components/messages/usage-panel.ts apps/kimi-code/test/tui/components/messages/usage-panel.test.ts --quiet
• pnpm --filter @moonshot-ai/kimi-code run test
• git diff --check

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

gen-docs is not needed here because the existing command docs already describe /usage; this only adds detail to the rendered usage panel.

AI Assistance Disclosure

I used Codex to review the changes, sanity-check the implementation against existing patterns, and help spot potential edge cases.

[changeset-bot]

🦋 Changeset detected

Latest commit: b412d97

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Minor

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