perf(skill-loader): add blocking discovery API with worker threads

Implement synchronous skill discovery using Node.js Worker Threads and Atomics.wait for blocking operations. Allows synchronous API access while leveraging async operations internally via dedicated worker thread.

Changes:
- blocking.ts: Main blocking discovery function using SharedArrayBuffer and MessagePort
- discover-worker.ts: Worker thread implementation for async skill discovery
- blocking.test.ts: Comprehensive test suite with BDD comments

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/features/opencode-skill-loader/blocking.ts

@@ -0,0 +1,62 @@
+import { Worker, MessageChannel, receiveMessageOnPort } from "worker_threads"
+import type { LoadedSkill, SkillScope } from "./types"
+
+interface WorkerInput {
+  dirs: string[]
+  scopes: SkillScope[]
+}
+
+interface WorkerOutputSuccess {
+  ok: true
+  skills: LoadedSkill[]
+}
+
+interface WorkerOutputError {
+  ok: false
+  error: { message: string; stack?: string }
+}
+
+type WorkerOutput = WorkerOutputSuccess | WorkerOutputError
+
+const TIMEOUT_MS = 30000
+
+export function discoverAllSkillsBlocking(dirs: string[], scopes: SkillScope[]): LoadedSkill[] {
+  const signal = new Int32Array(new SharedArrayBuffer(4))
+  const { port1, port2 } = new MessageChannel()
+  
+  const worker = new Worker(new URL("./discover-worker.ts", import.meta.url), {
+    workerData: { signal }
+  })
+
+  worker.postMessage({ port: port2 }, [port2])
+  
+  const input: WorkerInput = { dirs, scopes }
+  port1.postMessage(input)
+
+  const waitResult = Atomics.wait(signal, 0, 0, TIMEOUT_MS)
+
+  if (waitResult === "timed-out") {
+    worker.terminate()
+    port1.close()
+    throw new Error(`Worker timeout after ${TIMEOUT_MS}ms`)
+  }
+
+  const message = receiveMessageOnPort(port1)
+  
+  worker.terminate()
+  port1.close()
+
+  if (!message) {
+    throw new Error("Worker did not return result")
+  }
+
+  const output = message.message as WorkerOutput
+
+  if (output.ok === false) {
+    const error = new Error(output.error.message)
+    error.stack = output.error.stack
+    throw error
+  }
+
+  return output.skills
+}