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/model_registry.go

@@ -41,6 +41,7 @@ func DefaultModelRules() ModelRules {
 type DefaultModelRegistry struct {
 	models map[string]interface{}
 	rules  map[string]ModelRules
+	info   map[string]ModelInfo
 	mutex  sync.RWMutex
 }