feat: scope an action to the row it was invoked on

The five *_thread actions and their submenu are gone: the row's identity is
what decides the scope, so a second set of actions was a second answer to a
settled question. mark_thread_unread went with them, being the sixth entry in
the same submenu. tagSelected() loses its TagScope parameter, and
everySelectedRowHasTag() its own, so the direction and the write ask the same
question of the same object. ThreadListModel::scopeFor() and messageScopeFor()
are deleted; scopeForSelection() is the one resolver.

Labels name the scope. Archive, Delete, Restore, Spam, Important and the
unread toggle all say "thread" on a conversation row, and Delete, Restore and
Archive are ABSENT on a reply: a single reply cannot be removed from a
conversation.

Compose follows the same rule. Forward, Save, Reply-all and Reply without
quoting disappear on a conversation row, which shows no message to act on, and
Reply becomes "Reply to this thread": reply-all, quoting nothing, threaded off
the conversation's NEWEST message so the answer lands at its end rather than
forking the discussion at its opening post. That id is not in the model, since
an unexpanded conversation holds no nodes for its replies, so it comes from
resolveThreadMessages(); resolveQuery() states its newest-first sort rather
than inheriting notmuch's default.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012iDeN6C7y97nHYPvP6ST4L

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