feat(prompts-core): add package skeleton

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

packages/prompts-core/src/index.ts

@@ -0,0 +1,8 @@
+export type {
+  LoadedPrompt,
+  LoadPromptInput,
+  ModelVariant,
+  PromptSource,
+  RuntimeInjection,
+  VariantTable,
+} from "./types"