Moves five self-discovery handlers (--list-commands,
--info-cli-stats, --info-cli-categories, --info-cli-help,
--gen-completion) out of main.cpp into a new
cli_introspect.{hpp,cpp} module. All five auto-discover
commands by parsing printUsage's stdout via tmpfile capture,
so the surface stays self-describing as new flags are added.
Useful for shell completion scripts that re-exec the binary
at completion time, IDE plugins, and 'is there a flag for X?'
search workflows. main.cpp shrinks by 276 lines (2,298 to
2,022). --validate-cli-help stays inline because it needs
direct access to the static-local kArgRequired array.
