chore(quickjs): refine dynamic subagent prompt to trigger on workflow keyword and to improve iterative eval behavior (#602)

### Summary

Prompt change only, no runtime behavior change.

Reworks the quickjs code-interpreter subagent prompt to steer the
orchestrator toward an explore-then-distribute workflow. It explores
with its own tools first, hands subagents file paths instead of pasted
contents, reuses persisted REPL variables rather than re-pasting prior
eval output, and returns results via the last expression instead of
`console.log`. Also gates the REPL sandbox note so it only mentions the
`tools.*` namespace when PTC is actually exposed.

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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>