Add ADR-013: plugin catalog, curated shelf, lean per-site builds

Design record for distributing optional plugins across GE sites: a small
mandatory core plus a catalog of optional plugins, packaged as signed
versioned artifacts, served from a transport-agnostic read-only shelf (a
SharePoint-synced or sneakernet folder - untrusted either way because every
decision-bearing byte is signed), verified at adopt AND at every load and
migrate. Lean per-site builds stage only chosen plugins into the backend
image and SPA bundle.

Status PROPOSED. Grounds the design in the real loader/contract/migration/
frontend code and records defects to fix along the way (upgrade-all
migrating unadopted folders, enable-without-install, reverse-dep checks
blind to unloaded plugins, missing cycle detection and dependency closure,
hardcoded plugin imports in the SPA router). Honest on scope: the frontend
re-org is the long pole (one core-router change plus per-plugin relocation),
not a mechanical move. Phased 0-5 with schema-lean and runtime-JS delivery
explicitly deferred.

CLAUDE.md

@@ -22,6 +22,7 @@ Architecture decisions live in `docs/adr/`. Read those before making schema or c
 - ADR-010: Frontend plugin hook contract - ACCEPTED
 - ADR-011: Machines rename + modeltypes retyping - ACCEPTED
 - ADR-012: GE-Enforce manifest ownership in shopdb - ACCEPTED
+- ADR-013: Plugin catalog, curated shelf, and lean per-site builds - PROPOSED
 
 ## Coding convention