Rewrite #10 with refined interactive command design: bare-word REPL
fallback instead of ! prefix, result table assigned to _, same shape
as captured commands. Add #11 for Ctrl-C clearing the current line
instead of exiting the REPL.
Mark issues #02 (backtick lexing/parsing), #03 (command execution
runtime), and #04 (argv parsing) as resolved. Add new issues for
configuration (#08), programmable prompt (#09), and interactive
command execution (#10).
