Revert the bespoke __prompt()/__prompt2() function mechanism from issue
#09 and return to stock Lua's _PROMPT/_PROMPT2 globals (which already
support dynamic prompts via __tostring metatables). Set default
_PROMPT = "lush> " in doREPL if not already defined by config.
Replace static _PROMPT/_PROMPT2 with dynamic __prompt(exitcode)/__prompt2(exitcode)
functions. Fallback chain: __prompt() → _PROMPT → "> ". Install a default __prompt
that shows the current directory with ~ abbreviation. Track last exit code from the
REPL loop and pass it to the prompt function.
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).
