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/discover-worker.ts

@@ -0,0 +1,59 @@
+import { workerData, parentPort } from "worker_threads"
+import type { MessagePort } from "worker_threads"
+import { discoverSkillsInDirAsync } from "./async-loader"
+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 { signal } = workerData as { signal: Int32Array }
+
+if (!parentPort) {
+  throw new Error("Worker must be run with parentPort")
+}
+
+parentPort.once("message", (data: { port: MessagePort }) => {
+  const { port } = data
+
+  port.on("message", async (input: WorkerInput) => {
+    try {
+      const results = await Promise.all(
+        input.dirs.map(dir => discoverSkillsInDirAsync(dir))
+      )
+      
+      const skills = results.flat()
+      
+      const output: WorkerOutputSuccess = { ok: true, skills }
+      
+      port.postMessage(output)
+      Atomics.store(signal, 0, 1)
+      Atomics.notify(signal, 0)
+    } catch (error: unknown) {
+      const output: WorkerOutputError = {
+        ok: false,
+        error: {
+          message: error instanceof Error ? error.message : String(error),
+          stack: error instanceof Error ? error.stack : undefined,
+        },
+      }
+      
+      port.postMessage(output)
+      Atomics.store(signal, 0, 1)
+      Atomics.notify(signal, 0)
+    }
+  })
+})