Non-blocking issues surfaced while reviewing the :ZPack subcommand
refactor and its cmd_prefix deprecation shim.

commands.lua:
- complete_command strips the command word explicitly, so a bang-attached
  subcommand (:ZPack!load) no longer mis-parses and offers subcommand names
  at the plugin-name position
- the dispatcher rejects a bang on subcommands that ignore it
  (update/restore/clean), restoring the feedback the old :ZUpdate! E477 gave
- the dispatcher rejects extra positional args instead of joining them into
  a misleading 'Plugin "a b" not found in spec' error
- validate_name type-checks its input, so a non-string cmd_prefix/cmd_name
  no longer throws and aborts setup()
- setup_legacy threads the resolved cmd_name into legacy command descs and
  warnings instead of hardcoding :ZPack
- setup_legacy emits an error notice for an invalid cmd_prefix instead of
  returning silently
- a startup assert keeps SUB_ORDER in sync with the Sub table
- the cmd_prefix deprecation warning dedups once total, not once per command

tests:
- repurpose cmd_prefix_test.lua as an explicit legacy-shim test: rename the
  describe block, assert the deprecation warning fires, drop the rejection
  cases duplicated by cmd_name_test.lua, and fix delete_zpack_commands
  cleanup calls that passed the prefix as the cmd_name positional
- add dispatch_test.lua covering the bang/extra-arg/completion fixes
- deprecation_test asserts the warning dedups across different legacy commands
- drop the :ZPack-creation test duplicated from cmd_name_test.lua

see: zuqini/zpack.nvim#22