docs(backlog): item 136 fails deterministically, and names its own cause

Found incidentally while building item 152, by an agent that checked rather
than assumed: it ran test_mainwindow at the preceding commit in a throwaway
worktree and got the identical failure, so the signatures work is ruled out.

Records the assertion text, which is worth more than the flakiness history.
After the undo the message file is in neither cur nor new of the account
inbox, so the question narrows from "why does this race" to "where did the
file go", and the trash folder and the account root are the places to look
first. A move landing in the wrong folder is the mail-safety half of the
fork this entry already described, and it would present exactly this way.

The 70-second duration already recorded fits a QTRY_* waiting for a file
that is never going to appear.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEcn3u19xPqv6ggD15PG4c

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