docs: spec built-in filters as item 93, fold item 90 into it

Explaining item 90 to the user produced a reframing rather than a fix. The
buttons and the "more queries" menu are two different kinds of thing sharing
one mechanism: a filter narrows whatever the user is looking at and should
compose with the account dropdown, while a saved query is a self-contained
destination entitled to set the account itself.

Nothing ships as a default today, so the buttons are whatever the user pinned,
which the queries.json migration did to every [queries] entry. That drift is
the defect.

Item 93 ships four built-in filters, Unread, Inbox, Flagged and Sent, as
generated entries in the closed kQueryGenerators set that already exists for
Sent. The user's own pinned queries are unpinned rather than deleted once the
buttons are confirmed working.

Three findings from reading the code, all in the spec. A generator must answer
per account rather than having its all-accounts query wrapped in a scope, or
Sent becomes path:"a/**" and (path:"a/Sent/**" or path:"b/Sent/**"), which
returns the right rows only because path: is hierarchical. Sent is flat and
the other three are not, so the four match in scope and not in view mode. And
m_accountBox has no signal connected to it, which is now a decision rather
than an omission: changing the account runs nothing, the button is the verb.

Item 90's section moves to the closed file, kept in full because its cause and
the rules preview that motivated the reset are still true of the code.

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