-BLOCK was an alias of BLOCK, so the selected objects were always
converted to a block reference; a script doing -BLOCK then INSERT ended
up with an extra reference. -BLOCK is now its own command: the command
line erases the selection, and its JSON form defaults to keep "delete".
BLOCK (menu Make..., typed BLOCK) and the "block" JSON form still
convert by default.
Both forms ask 'Block "<name>" already exists. Redefine it? [Yes/No]
<N>:' when the name is in use; No asks for the name again.
Session::start resolves a dashed name to a command with that id (the
same change as #280's -ATTEDIT).
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
