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.

docs/adr/README.md

@@ -25,6 +25,7 @@ Each ADR captures a single architectural decision: the context, the decision its
 | [010](ADR-010-frontend-plugin-hooks.md) | Frontend plugin hook contract | ACCEPTED |
 | [011](ADR-011-machines-rename.md) | Machines rename + modeltypes retyping | ACCEPTED |
 | [012](ADR-012-geenforce-manifest-ownership.md) | GE-Enforce manifest ownership in shopdb | ACCEPTED |
+| [013](ADR-013-plugin-catalog-and-lean-builds.md) | Plugin catalog, curated shelf, and lean per-site builds | PROPOSED |
 
 ## Authoring