fix(run): accept - as the stdin script argument

[clawedassistant26]

The documented stdin form moshcode run - < script.mosh exits 1 with unknown option -.

Reproduce

$ echo 'say("hi from stdin")' | moshcode run -
moshcode run: unknown option -
$ echo $?
1

Cause

readScript() already handles the bare - (bin/moshcode.mjs:37):

if (!arg || arg === "-") return fs.readFileSync(0, "utf8"); // stdin (paste)

But the option parser rejects it first (bin/moshcode.mjs:289), because - starts with a dash and no positional has been seen yet:

else if (a.startsWith("-") && positional.length === 0) {
  console.error(`moshcode run: unknown option ${a}`);
  process.exit(1);
}

So the stdin path is unreachable from the CLI. It is documented in two places: README.md:164 (moshcode run - < script.mosh # pipe/paste from stdin) and the comment at bin/moshcode.mjs:56.

Fix

Exclude the bare - from the leading-dash check so it falls through to positional, where readScript() picks it up.

else if (a !== "-" && a.startsWith("-") && positional.length === 0) {

Verification

$ echo 'say("hi from stdin")' | node bin/moshcode.mjs run -
  💬 hi from stdin
$ echo $?
0

$ node bin/moshcode.mjs run --bogus
moshcode run: unknown option --bogus     # still exits 1

Added run - reads the script from stdin to test/run-options.test.mjs. It fails on master (not ok 88) and passes with the fix. Full suite: 146/146 pass, up from 145 baseline.