script/js executes its `code` as host JavaScript (via new Function), and a step's
`code` is a *resolved* input — so it could be written as `{ $from: <chat-step> }`,
turning model/API output into the body of the executed function (untrusted data
-> arbitrary host code execution). Pipeline validation now requires script/js
`code` to be a literal string: any $from/expression-sourced code is rejected.
Authoring a literal script/js step remains supported (the pipeline file is the
trust boundary, like a shell/npm script). Combined with "dry-run never executes
$js", this closes the path where untrusted text reaches the JS sink.
Adds regression tests: $from-sourced code rejected, literal code accepted,
dry-run does not execute $js, getByJsonPointer blocks prototype/inherited keys,
and concurrency clamps to the maximum.
https://claude.ai/code/session_017ZGQCjwNQF5Pz96gLUnnG1
qcq01083097