Yield completed-output retention work between bounded steps

[bb-slop-cop]

Human comments

What was wrong

The completed-output retention job passed the full 250-row limit into synchronous SQLite JSON scans and updates in one event-loop turn. Its generic asynchronous work frame also excluded that synchronous body from blocking-work attribution. The direct reproduction and root-cause report is https://get-bb.github.io/reports/issues/3196.html.

What changed

• Run completed-output retention in 10-row synchronous steps, yield between full steps, and preserve the existing 250-row per-path sweep cap.
• Return scan-progress information from the database helper so the server stops after a partial or empty step.
• Attribute every synchronous step with runEventLoopWorkSync.
• Isolate the existing destroyed-environment attribution test to the sweep it verifies.
• Add a regression test that observes intermediate retention progress and checks blocking-work attribution.

There are no wire, protocol, migration, schema, CLI, configuration, authentication, authorization, permission, packaging, or release changes.

How you verified

• The focused regression test failed on trusted origin/main in two clean checkouts because no intermediate event-loop progress was observable; it passes after the fix.
• pnpm exec turbo run test --filter=@bb/server -- test/services/periodic-sweeps.test.ts test/services/completed-output-yield.test.ts test/system/event-loop-stall-monitor.test.ts — 18 tests passed.
• pnpm exec turbo run test --filter=@bb/db — 448 tests passed.
• pnpm exec turbo run typecheck --filter=@bb/db --filter=@bb/server — 6 tasks passed.
• git diff --check passed.
• Changed-line count: 229 total text lines (198 additions, 31 deletions) across 5 files.

Fixes #3196

AGENT GENERATED

[SawyerHood]

Closing as stale: main replaced the targeted retention implementation with bounded output migration/sweep work that yields. This does not establish that every broader performance concern in #3196 is resolved.

Closing at Sawyer’s request following the relevance review against main at b9fdeec.

AGENT GENERATED