Historian child cleanup races late OpenCode part writes

[coleleavitt]

Summary

Magic Context deletes a successful magic-context-compartment child immediately after the prompt API returns. OpenCode can still have detached session writers persisting step/summary parts, so the late insert targets a deleted session_id/message_id and fails its foreign key.

Observed failure

Failed query: insert into "part" (...) values (...) on conflict (...) do update ...
params: ..., {"type":"step-start"}, ...

Observed sequence:

1. Historian child created and streamed for about five minutes.
2. Magic Context accepted/published the compartment and deleted the child.
3. OpenCode persisted a late step-start part for that child.
4. The insert failed because the child session/message rows were already gone.

Root cause

runHistorianPrompt() deleted successful children in finally. Prompt completion and even session.idle are not global quiescence barriers: OpenCode can fork detached summary/session writes.

Expected behavior

Never delete historian children on the prompt-return path. Retire them only through an age-gated maintenance sweep whose stale budget covers the configured timeout, outer retries, model-suggestion retry, and fallback chain. Preserve children when keep_subagents is enabled.

Regression requirements

• Successful run does not call session.status or session.delete inline.
• Recent historian children are never swept.
• Stale historian children are swept after the full configured attempt budget.
• Dreamer-disabled installations still run the sweep.
• Slow reasoning models receive at least a ten-minute default timeout.

[alfonso-magic-context]

Fixed on master, ships next release. Historian children are no longer deleted inline at publish — they ride the age-gated orphan sweep, whose stale threshold now composes the configured timeout, retries, fallback chain, and a detached-writer grace window, and the sweep runs even with the dreamer disabled. Your sequence diagnosis was exact; see also PR #337 (filed independently by @coleleavitt for the same defect) for the extraction credits.