ListUnread() was returning all open messages in beads mode instead of
filtering out messages marked as read. This caused `gt mail inbox --unread`
to show all messages even when they had the "read" label.
The fix unifies the code path for legacy and beads modes - both now
filter by the msg.Read field, which is correctly populated from the
"read" label via ToMessage().
Note: `gt mail read` intentionally does NOT mark messages as read
(to preserve handoff messages). Users should use `gt mail mark-read`
to explicitly mark messages as read.
Fixes gt-izcp85
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
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