fix(tui): stop /shell dropping quotes from the command it runs

[clawedassistant26]

The bug

In the mosh pit, /shell <cmd> re-joins the tokenized command line with single spaces before handing it to $SHELL -c:

await openShell(rest.length ? rest.join(" ") : null);

splitCommandLine() has already stripped the user's quotes and escapes at that point, so the shell re-parses the flattened string and splits it again. Any argument containing whitespace silently becomes several arguments.

/shell git commit -m "two words" runs git commit -m two words.

Reproduced on main

$ printf '/shell printf "[%s]\\n" "two words"\n' | node bin/moshcode.mjs
mosh ▸ · bash -c printf [%s]\n two words
[two]n[words]n

Two arguments instead of one, and the escape is gone too. The !cmd escape, which is the same feature, is correct:

$ printf '!printf "[%s]\\n" "two words"\n' | node bin/moshcode.mjs
mosh ▸ · bash -c printf "[%s]\n" "two words"
[two words]

The ! branch already documents why ("we take the raw remainder (not the tokenized parts) so quoting is preserved") — /shell just never got the same treatment.

The fix

src/tui.mjs only: added commandRemainder(line), which returns everything after the first word exactly as typed, and used it for the /shell and /sh branch. Both shell entrypoints now hand the same raw string to $SHELL -c.

Nothing else changes. The tokenized rest is still what native passthroughs like /coinpay and /ugig use — those spawn directly with an argv array, so their quoting was already correct and is untouched.

Tests

Two regression tests in test/tui.test.mjs, both driving the real TUI over stdin:

• a quoted argument survives /shell as one argument;
• /shell <cmd> and !<cmd> produce identical output for the same command.

Both fail on main (verified with git stash push -- src/: 6 pass / 2 fail) and pass with the fix. They skip on Windows, which has no POSIX shell for the probe command.

Full suite: node --test → 155 pass / 0 fail (153 before).