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>

tests/integration/test_deck_move_card_to_board.py

@@ -252,6 +252,42 @@ async def test_move_card_to_board_preserves_done_and_archived(
     assert after.archived is True, "archived state was lost during the move"
 
 
+async def test_move_card_to_board_preserves_assigned_users(
+    nc_client: NextcloudClient, two_boards_with_stacks: tuple
+):
+    """Assigned users carry over a cross-board move (the route doesn't touch them).
+
+    Deck's update route remaps labels on a board change but leaves the card's
+    user assignments untouched, so an assignee that exists on both boards stays
+    assigned.
+    """
+    source_board_id, source_stack_id, target_board_id, target_stack_id = (
+        two_boards_with_stacks
+    )
+
+    suffix = uuid.uuid4().hex[:8]
+    card = await nc_client.deck.create_card(
+        source_board_id, source_stack_id, f"Assigned {suffix}"
+    )
+    # The test user owns both temporary boards, so it is a valid assignee on each
+    await nc_client.deck.assign_user_to_card(
+        source_board_id, source_stack_id, card.id, nc_client.username
+    )
+    before = await nc_client.deck.get_card(source_board_id, source_stack_id, card.id)
+    assert before.assignedUsers, "Card should have an assignee before the move"
+
+    await nc_client.deck.move_card_to_board(
+        source_board_id=source_board_id,
+        source_stack_id=source_stack_id,
+        card_id=card.id,
+        target_board_id=target_board_id,
+        target_stack_id=target_stack_id,
+    )
+
+    after = await nc_client.deck.get_card(target_board_id, target_stack_id, card.id)
+    assert after.assignedUsers, "Assigned users were dropped during the move"
+
+
 async def test_move_card_to_board_rejects_stack_not_on_target_board(
     nc_client: NextcloudClient, two_boards_with_stacks: tuple
 ):