docs: add backlog items 15 to 17 from usage notes

Attachments turned out to be a real gap rather than a discoverability
one. MessageView creates the attachment bar and adds it to the pane, but
nothing ever populates it: m_attachmentBar appears nowhere else in the
codebase, so it has never displayed anything. MimeParser already
extracts attachments and Attachment::saveTo() already carries the
path-traversal guard, so the backend needs calling, not writing.

Item 16 makes delete a toggle, with the open question of what a mixed
selection should do. Item 17 needs a new worker call, since there is no
way to list tags today.

Also records that this document's numbering and the user's own notes
have diverged, so a reference to "item 13" stays resolvable.

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

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