Add !command syntax that runs commands with inherited terminal (no
capture). The lexer treats ! as a statement-starting token, reading
to end-of-line with the same interpolation/escape/pipe support as
backtick commands. The C function sets _ = {code=N, stdout="", stderr=""}.
Add split_pipeline() to split command strings on unquoted | and
exec_pipeline() to fork N children connected by inter-stage pipes.
Only the last stage's stdout/stderr are captured; middle stages'
stderr is inherited. Exit code comes from the last stage.
Redesign issue #10: bare-word commands now work in both scripts and
the REPL via a parser-level heuristic (identifier + non-exception-list
token → shell command). Add runtime fallback for string-arg syntax
(echo "hello"), double-dash flag handling, and classification examples.
Add issue #12 for path-based command execution (./script, /bin/ls, ~/bin/deploy).
Add testes/lush/commands-interactive.lua as a design playground covering
result table structure, exit codes, commands inside Lua blocks, _ behaviour,
runtime fallback, Lua variable shadowing, and interleaved Lua/shell.
Add test suite under testes/lush/ covering backtick commands, argv
parsing, ${} interpolation, and $NAME environment variables. Wire
them into testes/all.lua so they run with the full Lua 5.5 suite.
Skip /dev/full test in files.lua when the device doesn't exist
(macOS has no /dev/full).
