Moved into multica-platform

Multica's platform contracts are now one skill. Everything this skill used to carry — PR linking vs close intent, reading a linked PR's real state, custom properties, status side effects, sub-issues and stages, and finding who else is running — lives in:

multica-platform  →  references/issues.md

Load the multica-platform skill and open that file. Its routing table also names the reference for every other platform domain: mentions, agents, squads, autopilots, projects, runtimes, and skill import.