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/opencode/test/fixture/agent-plugin.constants.ts

@@ -0,0 +1,6 @@
+// Separate file because every export in `agent-plugin.ts` must be a function.
+export const PLUGIN_AGENT = {
+  name: "plugin_added",
+  description: "Added by a plugin via the config hook",
+  mode: "subagent",
+} as const