feat(prompts-core): add prompt loader

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 +1,61 @@
+import { parseFrontmatter } from "@oh-my-opencode/utils"
+import { readFile } from "node:fs/promises"
+import { join } from "node:path"
+import type { LoadedPrompt, LoadPromptInput, RuntimeInjection } from "./types"
+
+export class PromptFileNotFoundError extends Error {
+  readonly name = "PromptFileNotFoundError"
+
+  constructor(
+    readonly promptName: string,
+    readonly variant: string,
+    readonly filePath: string,
+    options?: ErrorOptions
+  ) {
+    super(`Prompt file not found for ${promptName}/${variant}: ${filePath}`, options)
+  }
+}
+
+export async function loadPrompt<TFrontmatter = Record<string, unknown>>(
+  input: LoadPromptInput
+): Promise<LoadedPrompt<TFrontmatter>> {
+  const filePath = join(input.source.baseDir, input.name, `${input.variant}.md`)
+  const content = await readPromptFile(input.name, input.variant, filePath)
+  const parsed = parseFrontmatter<TFrontmatter>(content)
+  const body = await applyRuntimeInjections(parsed.body, input.inject ?? [])
+
+  return {
+    frontmatter: parsed.data,
+    body,
+    hadFrontmatter: parsed.hadFrontmatter,
+    parseError: parsed.parseError,
+    filePath,
+  }
+}
+
+async function readPromptFile(promptName: string, variant: string, filePath: string): Promise<string> {
+  try {
+    return await readFile(filePath, "utf8")
+  } catch (error) {
+    if (error instanceof Error && getErrorCode(error) === "ENOENT") {
+      throw new PromptFileNotFoundError(promptName, variant, filePath, { cause: error })
+    }
+    throw error
+  }
+}
+
+async function applyRuntimeInjections(
+  body: string,
+  injections: readonly RuntimeInjection[]
+): Promise<string> {
+  let renderedBody = body
+  for (const injection of injections) {
+    renderedBody = renderedBody.replaceAll(injection.placeholder, await injection.resolver())
+  }
+  return renderedBody
+}
+
+function getErrorCode(error: Error): string | undefined {
+  if (!("code" in error)) return undefined
+  return typeof error.code === "string" ? error.code : undefined
+}