docs(backlog): close item 104, file item 166

Item 104 is fixed and its entry was wrong. It named mbsync's folder
patterns as the leading theory and concluded the cause was most likely
outside this repository; the reproduction put it at layer 3, in the
worker's own handle. The superseded theory is kept, since it would
produce a similar symptom and remains worth checking first in any future
report of this shape.

Two measurement errors from the diagnosis are recorded with it. A bare
`inbox` in a notmuch query is a free-text term rather than a tag term,
and the application generates `tag:inbox`; reading a message's tags
across every file matching a subject mixes several accounts' copies into
one answer. Each produced a confident wrong answer before it was caught.

Item 166 is new, found while setting up msmtp. The `post-new` hook's
sent-folder carve-out judges provenance by a file's path, but notmuch
deduplicates by Message-ID, so mail sent between two of the user's own
accounts is one message with a file in each. The carve-out matches the
sent copy and strips `inbox` from the message the recipient's inbox copy
also belongs to. Three options are laid out; the fix is a two-repo
change and the hook runs unattended on live mail, so it needs a decision
rather than a patch.

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