feat(resolvemcp): add model descriptions, usage guide and catalogue export

- modelregistry: ModelInfo (description, purpose, tags, column docs) with
  external JSON loader, Describer fallback and gorm/bun/comment tag support
- resolvemcp: surface descriptions in list_tables and describe_table, send a
  usage guide as MCP server instructions, add package docs
- add BuildCatalog/ExportCatalog to write a JSON or Markdown API catalogue
- document the descriptions map and catalogue in the README

pkg/modelregistry/doc.go

@@ -0,0 +1,32 @@
+// Package modelregistry is the shared catalogue of the Go model structs that the
+// ResolveSpec front ends (resolvespec, restheadspec, websocketspec, mqttspec,
+// resolvemcp, ...) expose as database entities.
+//
+// A registry maps a model name ("schema.entity") to a struct type and holds:
+//   - ModelRules: which operations (read/create/update/delete, public or not)
+//     are allowed, and whether security checks are disabled.
+//   - ModelInfo: optional documentation (description, purpose, tags, per-column
+//     descriptions) meant for humans and AI agents. It never affects queries or
+//     permissions.
+//
+// Register models on a registry created with NewModelRegistry, or through the
+// package-level functions that use the default registry:
+//
+//	reg := modelregistry.NewModelRegistry()
+//	_ = reg.RegisterModelWithRules("public.users", User{}, modelregistry.DefaultModelRules())
+//	reg.SetModelInfo("public.users", modelregistry.ModelInfo{
+//		Description: "Application accounts",
+//		Purpose:     "Look up who a person is; never store credentials here",
+//		Columns:     map[string]string{"email": "Login address, unique"},
+//	})
+//
+// Descriptions come from, in priority order:
+//  1. ModelInfo set with SetModelInfo or loaded from an external JSON map with
+//     LoadModelInfoFile (the map can be maintained outside the Go code).
+//  2. The model's Describer (ModelDescription() string) for the description.
+//  3. Struct tags, read per column by FieldComment: comment, note, desc or
+//     description tags, then "comment:" inside the gorm or bun tag.
+//
+// Models must be non-pointer structs; pointers, slices and arrays of structs are
+// unwrapped on registration. All registry methods are safe for concurrent use.
+package modelregistry