feat(prompts-core): add public prompt loader types

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

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

packages/prompts-core/src/types.ts

@@ -0,0 +1,51 @@
+export interface VariantTest {
+  test: (input: { modelID?: string; agentName?: string }) => boolean
+}
+
+export interface VariantFallback {
+  fallback: true
+}
+
+export type VariantDefinition = VariantTest | VariantFallback
+
+export interface VariantTable {
+  [variantKey: string]: VariantDefinition
+}
+
+export interface ResolveVariantInput {
+  modelID?: string
+  agentName?: string
+  variants: VariantTable
+}
+
+export interface RuntimeInjection {
+  placeholder: string
+  resolver: () => string
+}
+
+export interface LoadPromptInput {
+  source: string
+  name: string
+  variant: string
+  inject?: readonly RuntimeInjection[]
+}
+
+export interface LoadedPrompt {
+  body: string
+  frontmatter: Record<string, unknown>
+  resolvedPath: string
+}
+
+export class PromptNotFoundError extends Error {
+  constructor(message: string) {
+    super(message)
+    this.name = "PromptNotFoundError"
+  }
+}
+
+export class NoMatchingVariantError extends Error {
+  constructor(message: string) {
+    super(message)
+    this.name = "NoMatchingVariantError"
+  }
+}