docs: close item 103 and tick the delete-to-trash plan

Every step of the plan is done. Item 103's section moves to the closed file on
the same commit, per this repo's own rule, with its outcome recorded: what was
built, the ten defects hand testing found that the suite did not, and the two
process gaps closed alongside them.

The fact worth carrying forward is the one that damaged real mail. Under
mbsync's Create Both, a wrongly named origin folder propagates to the mail
server, so any code composing a folder name reaches the server whether it means
to or not.

Item 118, emptying the trash, remains deferred at the user's request.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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