feat: 品牌替换 + 启动优化 + AGENTS.md 模板定制

- 品牌替换：OpenCode/opencode → AirCoding/aircoding（16+ 文件）
- Logo ASCII art：修复 left/right 行数不匹配导致的启动崩溃
- 启动诊断：添加 OPENCODE_PRINT_TIMING 计时探针
- dev 模式默认 --pure 跳过外部插件加载
- AGENTS.md 模板：追加 AirCoding 多 Agent 专项段落
- architect prompt + plugin：强化 AGENTS.md 产出验证

packages/core/test/fixture/tmpdir.ts

@@ -0,0 +1,25 @@
+import fs from "fs/promises"
+import { tmpdir as osTmpdir } from "os"
+import path from "path"
+
+export const tmpdir = async () => {
+  const dir = await fs.realpath(await fs.mkdtemp(path.join(osTmpdir(), "opencode-core-test-")))
+  return {
+    path: dir,
+    async [Symbol.asyncDispose]() {
+      await remove(dir)
+    },
+  }
+}
+
+async function remove(dir: string, retries = 30): Promise<void> {
+  try {
+    await fs.rm(dir, { recursive: true, force: true })
+  } catch (error) {
+    if (retries === 0 || !error || typeof error !== "object" || !("code" in error) || error.code !== "EBUSY")
+      throw error
+    Bun.gc(true)
+    await Bun.sleep(100)
+    return remove(dir, retries - 1)
+  }
+}