Flaky e2e: TestJoinHistoryFolderPlain finds no assistant entry in session_joined history #392

Problem

Relay main push CI at 6730e9f (run 38444948647, e2e) failed:
g1_history_folder_test.go:113: session_joined history has no assistant entry with the reply; history: [{"role":"user","content":"hello history"}]

The same tree passed the whole e2e job in PR #390's last run. Run alone on the test machine, TestJoinHistoryFolderPlain and TestJoinHistoryFolderWithSpaceAndDot pass 20 of 20 at 6730e9f and at 7c3c45c (go test -race -count=10). It shows only under the full parallel e2e load.

Root cause

Not found in the 30-minute pass. The fake claude writes the user and assistant transcript lines before it reports the result (e2e/fakes/fakeagent/claude.go:79), and the test rejoins after the result, so the history should hold both. The user line was there and the assistant line was not. A partly flushed transcript read, or a history merge that drops the assistant entry under load, would both fit; neither is shown.

Fix

Next: reproduce under load (-count with -parallel 32 and the rest of ./features), then fix the wait or the read.

ship-blocker: no

Found while working barelyworkingcode/eve#424.