docs(plugin-dev): 注明 timer 已外置，并写明「先装插件再升级内核」的顺序约束

timer 移出内置插件后，升级路径上有个静默故障点：若先升级内核、后装 timer 插件，
就会有一段窗口期三个 timer 工具全部不存在 —— 不报错、日志无线索，只是 LLM
视野里工具没了，表现为「定时提醒功能不见了」，极难归因。

在 zh/en 两份 PLUGIN_DEV.md 的「内置插件」表下补注记（原 Timer 示例行就删在这里，
正是读者会产生疑问的位置）：
  - timer 已改为外部 hmap 插件提供，三个工具与 timer 输入通道均由它注册
  - 升级顺序：先装 timer 插件，再升级内核
  - 外部版是严格超集：原内置版只有 timer_set 且重启丢在途倒计时，
    外部版增加 timer_list / timer_cancel 并跨重启保留待触发列表

assets/docs 是纯文档，无 go:embed、无代码引用，改动不影响构建。

(cherry picked from commit 62332562a80165b6e86af77315c07d18e743e7fa)

assets/docs/en/PLUGIN_DEV.md

@@ -839,6 +839,19 @@ Internal: records are stored in SQLite `disabled_plugins` table (`name`, `disabl
 | CLI | `internal/plugins/cli/` | Unix socket listener + synchronous request-response |
 | WebUI | `internal/plugins/webui/` | HTTP service + dependency injection |
 
+> **timer has been moved out of the built-in plugins** and is now provided as an external
+> hmap plugin (all three tools `timer_set` / `timer_list` / `timer_cancel` and the `timer`
+> input channel are registered by the external plugin).
+>
+> ★ **Upgrade order**: install the timer plugin *before* upgrading the kernel. Doing it the
+> other way round leaves a window where none of the three timer tools exist — and it
+> **fails silently**: no error, the tools simply vanish (the LLM cannot see them, which
+> surfaces as "reminders are gone" with nothing in the logs to explain it).
+>
+> The external version is also a strict superset: the built-in one only had `timer_set`, and
+> in-flight countdowns were lost on restart; the external one adds `timer_list` / `timer_cancel`
+> and persists pending timers across restarts.
+
 ---
 
 *Want to understand the project goals? See [OVERVIEW.md](OVERVIEW.md).*