opsx:clarify

Reduce ambiguity in an OpenSpec change's planning artifacts before implementation. Scan proposal.md and design.md for gaps using a structured taxonomy, ask at most 5 targeted questions one at a time (each with a built-in recommendation), and write answers back into the artifacts incrementally.

Does NOT write code, modify tasks.md, or replace /opsx:explore.

Step 1: Change Selection

If a change name was passed as an argument, use it directly. Announce: "Using change: <name>."