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"

packages/prompts-core/src/types.test.ts

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+import { describe, expect, test } from "bun:test"
+import type { LoadPromptInput, LoadedPrompt, PromptSource, RuntimeInjection } from "./types"
+
+describe("prompt core types", () => {
+  test("#given loader input shape #then accepts source name variant and runtime injections", () => {
+    const source: PromptSource = { baseDir: "/tmp/prompts" }
+    const injection: RuntimeInjection = { placeholder: "{X}", resolver: () => "Y" }
+
+    const input = {
+      source,
+      name: "test",
+      variant: "default",
+      inject: [injection],
+    } satisfies LoadPromptInput
+
+    expect(input.source.baseDir).toBe("/tmp/prompts")
+    expect(input.inject[0]?.placeholder).toBe("{X}")
+  })
+
+  test("#given loaded prompt shape #then carries frontmatter and rendered body", () => {
+    const loaded = {
+      frontmatter: { title: "Fixture" },
+      body: "Prompt body",
+      hadFrontmatter: true,
+      parseError: false,
+      filePath: "/tmp/prompts/test/default.md",
+    } satisfies LoadedPrompt<{ readonly title: string }>
+
+    expect(loaded.frontmatter.title).toBe("Fixture")
+    expect(loaded.body).toBe("Prompt body")
+  })
+})

packages/prompts-core/src/types.ts

@@ -0,0 +1,35 @@
+export type ModelVariant =
+  | "default"
+  | "gpt"
+  | "gemini"
+  | "kimi"
+  | "glm"
+  | "planner"
+  | "opus-4-7"
+  | "minimax"
+
+export type PromptSource = {
+  readonly baseDir: string
+}
+
+export type RuntimeInjection = {
+  readonly placeholder: string
+  readonly resolver: () => string | Promise<string>
+}
+
+export type LoadPromptInput = {
+  readonly source: PromptSource
+  readonly name: string
+  readonly variant: string
+  readonly inject?: readonly RuntimeInjection[]
+}
+
+export type LoadedPrompt<TFrontmatter = Record<string, unknown>> = {
+  readonly frontmatter: TFrontmatter
+  readonly body: string
+  readonly hadFrontmatter: boolean
+  readonly parseError: boolean
+  readonly filePath: string
+}
+
+export type VariantTable = Readonly<Record<string, PromptSource>>