cube-harness

First-pass triage for a proposed change to cube-harness's shared surface (an RFC / OpenSpec change / core-API request). Separates the contributor's real need from the mechanism they proposed, finds the smallest thing that serves it, and produces an advisory verdict (RESHAPE / DECLINE / ESCALATE / ACCEPT — or ROUTE-UPSTREAM) plus a contributor-facing reply. A pedagogical friction step, not a gate. Accepts a GitHub PR URL, an `openspec/changes/<name>/` dir, or pasted proposal text. Invoke as `/gatekeep-rfc <arg>`.