When users forget the -i flag and run 'bd import file.jsonl', the command
would silently read from stdin instead of the file, resulting in '0 created,
0 updated' output that was misleading.
Root cause: The import command doesn't validate positional arguments, so
'bd import file.jsonl' would be interpreted as 'bd import' with an ignored
argument, reading empty stdin.
Fix: Add validation at the start of the import command to detect positional
arguments and show a helpful error message with the correct syntax.
Closes bd-77gm
196ce3a6f9