docs(twister): changeset for the NewNote.unread default correction

The branch corrects a doc comment that stated omitting `unread` leaves a
thread's read state alone. It does not, and authors relying on that reading
ship notes that silently mark threads unread. Record it in the changelog so
the correction reaches consumers.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GcnBUEdW86ovpv1pT2d3b3

Author: KrisBraun

.changeset/new-note-unread-default.md

@@ -0,0 +1,10 @@
+---
+"@plotday/twister": patch
+---
+
+Fixed: corrected the documented default for `NewNote.unread`.
+
+Omitting the flag was described as "leave read state alone". It is not — attaching
+a note marks its thread unread for every recipient except the note's author, and
+there is no outcome that leaves read state untouched. A note that should not raise
+unread must pass an explicit `false`.