feat(prompts-core): load markdown prompts with injection

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

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

.omo/evidence/pr1-prompts-core/loader.green.log

@@ -0,0 +1,14 @@
+bun test v1.3.12 (700fc117)
+
+packages/prompts-core/src/loader.test.ts:
+S8 actual: {"body":"Hello {PLACEHOLDER}, this is a test.\n","frontmatter":{"description":"test fixture"},"resolvedPath":"/Users/yeongyu/local-workspaces/omo-wt/feat/prompts-core-foundation/packages/prompts-core/src/__fixtures__/demo/default.md"}
+S9 actual: Prompt not found at /Users/yeongyu/local-workspaces/omo-wt/feat/prompts-core-foundation/packages/prompts-core/src/__fixtures__/missing/default.md
+S10 actual: Hello Y, this is a test.
+
+S11 actual: HeLLo Y, this is a demo.
+
+
+ 4 pass
+ 0 fail
+ 5 expect() calls
+Ran 4 tests across 1 file. [43.00ms]

.omo/evidence/pr1-prompts-core/s10.green.log

@@ -0,0 +1,14 @@
+bun test v1.3.12 (700fc117)
+
+packages/prompts-core/src/loader.test.ts:
+S8 actual: {"body":"Hello {PLACEHOLDER}, this is a test.\n","frontmatter":{"description":"test fixture"},"resolvedPath":"/Users/yeongyu/local-workspaces/omo-wt/feat/prompts-core-foundation/packages/prompts-core/src/__fixtures__/demo/default.md"}
+S9 actual: Prompt not found at /Users/yeongyu/local-workspaces/omo-wt/feat/prompts-core-foundation/packages/prompts-core/src/__fixtures__/missing/default.md
+S10 actual: Hello Y, this is a test.
+
+S11 actual: HeLLo Y, this is a demo.
+
+
+ 4 pass
+ 0 fail
+ 5 expect() calls
+Ran 4 tests across 1 file. [43.00ms]

.omo/evidence/pr1-prompts-core/s11.green.log

@@ -0,0 +1,14 @@
+bun test v1.3.12 (700fc117)
+
+packages/prompts-core/src/loader.test.ts:
+S8 actual: {"body":"Hello {PLACEHOLDER}, this is a test.\n","frontmatter":{"description":"test fixture"},"resolvedPath":"/Users/yeongyu/local-workspaces/omo-wt/feat/prompts-core-foundation/packages/prompts-core/src/__fixtures__/demo/default.md"}
+S9 actual: Prompt not found at /Users/yeongyu/local-workspaces/omo-wt/feat/prompts-core-foundation/packages/prompts-core/src/__fixtures__/missing/default.md
+S10 actual: Hello Y, this is a test.
+
+S11 actual: HeLLo Y, this is a demo.
+
+
+ 4 pass
+ 0 fail
+ 5 expect() calls
+Ran 4 tests across 1 file. [43.00ms]

.omo/evidence/pr1-prompts-core/s8.green.log

@@ -0,0 +1,14 @@
+bun test v1.3.12 (700fc117)
+
+packages/prompts-core/src/loader.test.ts:
+S8 actual: {"body":"Hello {PLACEHOLDER}, this is a test.\n","frontmatter":{"description":"test fixture"},"resolvedPath":"/Users/yeongyu/local-workspaces/omo-wt/feat/prompts-core-foundation/packages/prompts-core/src/__fixtures__/demo/default.md"}
+S9 actual: Prompt not found at /Users/yeongyu/local-workspaces/omo-wt/feat/prompts-core-foundation/packages/prompts-core/src/__fixtures__/missing/default.md
+S10 actual: Hello Y, this is a test.
+
+S11 actual: HeLLo Y, this is a demo.
+
+
+ 4 pass
+ 0 fail
+ 5 expect() calls
+Ran 4 tests across 1 file. [43.00ms]

.omo/evidence/pr1-prompts-core/s9.green.log

@@ -0,0 +1,14 @@
+bun test v1.3.12 (700fc117)
+
+packages/prompts-core/src/loader.test.ts:
+S8 actual: {"body":"Hello {PLACEHOLDER}, this is a test.\n","frontmatter":{"description":"test fixture"},"resolvedPath":"/Users/yeongyu/local-workspaces/omo-wt/feat/prompts-core-foundation/packages/prompts-core/src/__fixtures__/demo/default.md"}
+S9 actual: Prompt not found at /Users/yeongyu/local-workspaces/omo-wt/feat/prompts-core-foundation/packages/prompts-core/src/__fixtures__/missing/default.md
+S10 actual: Hello Y, this is a test.
+
+S11 actual: HeLLo Y, this is a demo.
+
+
+ 4 pass
+ 0 fail
+ 5 expect() calls
+Ran 4 tests across 1 file. [43.00ms]

packages/prompts-core/src/loader.ts

@@ -1,9 +1,38 @@
-import type { LoadedPrompt, LoadPromptInput } from "./types"
+import { parseFrontmatter } from "@oh-my-opencode/utils"
+import { readFile } from "node:fs/promises"
+import { resolve } from "node:path"
+
+import { PromptNotFoundError, type LoadedPrompt, type LoadPromptInput } from "./types"
+
+function hasErrorCode(error: unknown, code: string): boolean {
+  if (!(error instanceof Error)) return false
+  if (!("code" in error)) return false
+  return error.code === code
+}
+
+export async function loadPrompt(input: LoadPromptInput): Promise<LoadedPrompt> {
+  const resolvedPath = resolve(input.source, input.name, `${input.variant}.md`)
+  let content: string
+
+  try {
+    content = await readFile(resolvedPath, "utf8")
+  } catch (error) {
+    if (hasErrorCode(error, "ENOENT")) {
+      throw new PromptNotFoundError(`Prompt not found at ${resolvedPath}`)
+    }
+    throw error
+  }
+
+  const parsed = parseFrontmatter(content)
+  let body = parsed.body
+
+  for (const injection of input.inject ?? []) {
+    body = body.replaceAll(injection.placeholder, injection.resolver())
+  }
 
-export async function loadPrompt(_input: LoadPromptInput): Promise<LoadedPrompt> {
   return {
-    body: "__red__",
-    frontmatter: {},
-    resolvedPath: "__red__",
+    body,
+    frontmatter: parsed.data,
+    resolvedPath,
   }
 }