Add an optional `entityRef` (string, format: uri) to education, volunteering, certification, course, and honor, mirroring the field already on id.sifa.profile.position. It carries a portable, app-neutral organization identifier -- a Wikidata Q-ID URI, ROR URI, or LEI URI -- written when a user picks an org from a resolver dropdown, and absent for free-text entries. This extends deterministic org resolution beyond career positions to the institutions, organizations, and issuers referenced across the profile, resolving to a canonical entity with zero PDS rewrites when those orgs later claim an ATproto account (#159). Additive, backward-compatible field -> patch bump 0.8.3 -> 0.8.4.