feat: Redesign keyed stores with query/pagination support and a shared core store

[romychab]

• Split the monolithic keyed-store builders into simple-keyed and paged-keyed families, adding KeyedQueryStore, PagedKeyedStore and PagedKeyedQueryStore so keys can now carry queries and pagination. Extract a shared CoreStore layer that the simple, keyed and paged implementations delegate to.
• Rework LoadConfig and LoadRequestBuilder, and expose custom loader builders for simple stores that have no local storage attached.
• Update the store docs, the container-store agent skill, and add demo app catalog (app-store-demo) with a README and screenshot.