workflow: a card re-matched into another workflow leaves no trace in the source graph's log #1668

Implementation: FlyBot (ours) — PR follows within 24h. Do not pick up without a pickup-ok comment on this issue.

Written-by: FlyBot Lead
Claimed-by: FlyBot Lead
Claim-Id: 803225b92a644bd281b7c460cff02299
Branch: dev/3-reroute-source-log
Base: 6cad756
At: 2026-10-29T09:04:07Z

Implementation: FlyBot (ours) — PR follows within 24h. Do not pick up without a pickup-ok comment.

Written-by: FlyBot Lead

What this adds

When a card held by one workflow is re-matched into another (#1647), only the target side gets a log line: the source workflow's state.logs does not change at all. Someone watching that source graph sees a held card vanish with no explanation — exactly the step users are most likely to read as "the task was lost".

Current behaviour

• plugins/workflow/src/service.js, the re-route branch of ingest (around :1508-1532) writes only on the target side; beginRun (:790-800) pushes "Task assigned · …" and "Started · …" onto the target.
• Probe against the real service (product audit, live main): Alpha (trigger weekly) holds t-1; then ingest({…, workflowId: Beta}) → {ok:true, matched:true}; Alpha messages 4 → 4 (+0), heldTasks ["t-1"] → []; Beta 1 → 4 (+3).

Expected

The source workflow gets exactly one system line naming the task and the destination, in the same mutation (no extra write()), so the reroute is visible where the card disappeared.

Scope

• plugins/workflow/src/service.js: in the re-route branch, push one message onto the source side — key: 'Task re-matched · {task} → {to}', zh fallback 任务已改投 · {task} → {to}. About 6 lines.
• New test plugins/workflow/test/claim-reroute-log.test.js.
• One sentence in docs/subsystems/workflow.md (or a Docs-not-needed: reason if the doc count stays at 0).

Acceptance:

1. after a reroute the source messages has exactly one more entry and it carries both t-1 and the destination workflow name;
2. the target side is unchanged, entry for entry, versus current main;
3. the non-reroute path of #1647 (a claim that simply matches) appends 0 entries, and the on-disk bytes + mtime are unchanged;
4. already: true appends 0;
5. MAX_MESSAGES = 200 behaviour unchanged.

Not in scope: "what a graph save changed" (that is the separate backlog item P-136, same push()/state.logs helpers but a different event — do not fold it in); any client.js change.