test(prompts-core): cover prompt loader scenarios

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

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

packages/prompts-core/src/loader.ts

@@ -0,0 +1,9 @@
+import type { LoadedPrompt, LoadPromptInput } from "./types"
+
+export async function loadPrompt(_input: LoadPromptInput): Promise<LoadedPrompt> {
+  return {
+    body: "__red__",
+    frontmatter: {},
+    resolvedPath: "__red__",
+  }
+}