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/specs/v2/notifications.md

@@ -0,0 +1,13 @@
+# TUI Notifications Default
+
+Problem:
+
+- v1 defaults `attention.enabled` to `false`
+- users can opt in with `attention.enabled = true`
+- v2 should make core TUI notifications a default behavior
+
+## v2 Target
+
+Flip `attention.enabled` to `true` by default in v2.
+
+Keep `attention.enabled = false` as the explicit opt-out.