docs(deck): document assignedUsers preservation on cross-board move

Round-6 review: the docstrings listed preserved fields but omitted
assignedUsers. Verified empirically (Deck 1.15.9) that the update route's
board-change handling only remaps labels and leaves user assignments
untouched, so assignees carry over. Documented in both the client and MCP
tool docstrings, with the caveat that an assignee lacking access to the
target board stays assigned but cannot act on the card. Added
test_move_card_to_board_preserves_assigned_users to lock it in.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/client/deck.py

@@ -437,8 +437,11 @@ class DeckClient(BaseNextcloudClient):
         the same-titled label on the destination board, or cloning it there
         when the user has board-manage permission — instead of leaving orphaned
         labels behind. This mirrors Deck's native "Move/copy card" action. Card
-        identity (id, comments, attachments), ``archived`` state and the due
-        date are preserved.
+        identity (id, comments, attachments), ``archived`` state, the due date
+        and ``assignedUsers`` are preserved — the move does not re-validate
+        assignees against the destination board, so an assigned user without
+        access to the target board stays assigned but may not be able to act on
+        the card.
 
         The internal ``/apps/deck/cards/{cardId}`` route is used: it reads the
         target ``stackId`` from the body (the board/stack-scoped API route