[ci-doctor] CI Test Failure — Run #29057896142 (bun-version: latest regression)

## Summary

The `Test & Lint` job failed at the `Test` step (`bun test --coverage ./tests/`) on commit `7c5ea995`, which merged PR #410 ("Improve Evergreen CI diagnostics"). Only workflow files were changed — no source or test code was modified.

## Failure Details
- **Run**: [29057896142](https://github.com/githubnext/tsb/actions/runs/29057896142)
- **Commit**: `7c5ea995df0694314856f08440316b53004183d9`
- **Trigger**: push (merge of PR #410)
- **Failed Step**: `bun test --coverage ./tests/` (ran for ~4 seconds)
- **Previous successful run**: [29052744071](https://github.com/githubnext/tsb/actions/runs/29052744071) on commit `a7269d37`

## Root Cause Analysis

**No source code or test files were changed** between the successful and failing runs — only workflow files (`.github/workflows/evergreen.lock.yml`, `.github/workflows/evergreen.md`, `.github/workflows/shared/evergreen/repo-policy.md`).

The most likely cause is that `bun-version: latest` in `ci.yml` pulled a **new bun version** between the two runs, which introduced a regression in the test runner or coverage reporting.

This is a **recurring pattern** — an identical failure occurred previously (see #307).

## Failed Jobs and Errors

| Job | Step | Result |
|-----|------|--------|
| Test & Lint | Test | ❌ failure |
| Test & Lint | Type check | ✅ success |
| Test & Lint | Lint | ✅ success |

CI logs are not publicly accessible (require admin), so the exact test failure message is unavailable.

## Investigation Findings

1. Only `.github/workflows/` files changed — code is unchanged
2. The `Test` step ran for ~4 seconds (23:38:38–23:38:42), which is abnormally short for a full test suite
3. `Build` and cross-validation steps were skipped (they depend on `test` passing)
4. This is at least the third occurrence of this pattern (see also #307)

## Recommended Actions

- [ ] **Pin bun version** in `.github/workflows/ci.yml`: change `bun-version: latest` → `bun-version: "1.2.x"` (or the last known-good version)
- [ ] **Retry the failed run** — a re-run may succeed if the issue is a transient bun release
- [ ] **Check bun release notes** for any test runner or coverage changes between the last successful and current bun version

## Prevention Strategies

1. Pin bun to a specific version in all CI workflows; update deliberately after reviewing release notes
2. Consider adding a `continue-on-error: false` with explicit bun version check
3. Add a note to AGENTS.md reminding agents that `bun-version: latest` is not allowed in CI

## AI Team Self-Improvement

```
## CI Stability: Bun Version Pinning (CRITICAL)
- NEVER use `bun-version: latest` in CI workflow files (.github/workflows/*.yml).
Always pin to a specific version, e.g. `bun-version: "1.2.15"`.
- When modifying any CI workflow, verify that `bun-version` is pinned and not set to `latest`.
- Recurring test failures where only workflow files changed are almost always caused by
bun-version: latest pulling a new release with a regression. Recommend pinning immediately.
```

## Historical Context

- **Issue #307** (May 2026): Same pattern — `bun test` CI failure after autoloop merge, only workflow or unrelated files changed, tests pass locally. Root cause identified as `bun-version: latest`.
- This is the third known occurrence of this pattern in this repository.

> Generated by [CI Failure Doctor](https://github.com/githubnext/tsb/actions/runs/29057979379) · 326.4 AIC · ⌖ 11 AIC · ⊞ 28.1K · [◷](https://github.com/search?q=repo%3Agithubnext%2Ftsb+is%3Aissue+%22gh-aw-workflow-call-id%3A+githubnext%2Ftsb%2Fci-doctor%22&type=issues)
>

Add this agentic workflows to your repo

To install this agentic workflow, run

```
gh aw add githubnext/agentics/workflows/ci-doctor.md@c7d030cd6d4607b90d9ac3ffc8b24aff4f251632
```

.github/workflows/ci.yml から始め、失敗した実行 29057896142 と、その前に成功した実行 29052744071 を比較し、Test ステップで使用されている Bun のバージョンに注目してください。説明されているとおりに設定された Bun のバージョンを固定し、その後 workflow を再実行して、`bun test --coverage ./tests/` が正常に完了することを確認してください。

索引モデルが issue の本文から書いたものです。