chore: provide agent instructions for vitest

[rexxars]

Description

Getting a bit sick of claude re-running the entire test suite just to get more/less context - so giving these instructions and config a try. Will leave this open until I've actually tested that it works, but makes it easier to pull into branches where I want to give it a try.

What to review

Testing

[claude]

Claude finished @rexxars's task in 26s —— View job

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Review

The changes look good overall. Two observations:

vitest.config.ts:3 — CODEX_CI env var detection is included for non-Claude agents. Fine if intentional, but Claude.md only mentions CLAUDECODE. If you only want Claude to write JSON output, remove CODEX_CI from the check. If you want all agent environments to benefit, keep it.

Claude.md line 179 — The change from expect(error).toBeUndefined() to if (error) throw error in the example code is a recommended pattern change. The new form gives better failure messages (rethrows the actual error vs. generic assertion message) — this is an improvement, just noting it's a behavior change in the documented pattern, not only formatting.

No blocking issues.

[github-actions]

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Metric	Coverage
Statements	81.8% (±0%)
Branches	70.9% (±0%)
Functions	80.0% (±0%)
Lines	82.2% (±0%)