feat: add managed system prompt optimization

Add an opt-in OpenCode plugin that replaces the built-in provider behavioral prompt with a minimal identity while preserving environment, project, MCP, skill, history, and tool context.

Track the active agent per session and apply the transform only to build and plan. Keep plan/build mode reminders and permission enforcement owned by OpenCode, leave all other agents untouched, and fail safely when the expected prompt boundary is absent.

Expose the feature in Behavior settings with localized guidance, explicit Save + Reload application, settings search integration, persisted boolean validation, and managed-runtime lifecycle composition that does not load the plugin while disabled or on external OpenCode servers.

Document the runtime contract and cover plugin materialization, config preservation, build/plan selection, agent switching, unknown prompt formats, and settings sanitization.

packages/ui/src/lib/i18n/messages/zh-TW.settings.ts

@@ -543,6 +543,11 @@
   'settings.behavior.page.warning.title': '全域規則會與專案規則合併使用',
   'settings.behavior.page.warning.description': '此處所做的變更會更新 {path}。當專案級 AGENTS.md 存在時，OpenCode 也會包含它。',
   'settings.behavior.page.section.systemPrompt': '全域 AGENTS.md',
+  'settings.behavior.page.section.systemPromptOptimization': '系統提示詞最佳化',
+  'settings.behavior.page.systemPromptOptimization.enable': '最佳化系統提示詞大小',
+  'settings.behavior.page.systemPromptOptimization.enableAria': '最佳化 OpenCode 系統提示詞大小',
+  'settings.behavior.page.systemPromptOptimization.info': '預計可將 build 和 plan 代理程式的系統提示詞縮減約 40%，其他代理程式不會變更。這可能會移除覆寫 build 或 plan 的自訂定義，因此此類工作流程請勿啟用。重新啟動 OpenCode 後套用變更。',
+  'settings.behavior.page.systemPromptOptimization.restarting': '正在重新啟動 OpenCode 以套用系統提示詞最佳化…',
   'settings.behavior.page.field.systemPromptPlaceholder': '你是一個有幫助的 AI 助助理...\n\n在此設定 AI 在所有工作階段與服務提供者中都必須遵守的絕對行為規則。',
   'settings.behavior.page.section.responseStyle': '回覆風格',
   'settings.behavior.page.responseStyle.tooltip': '啟用後，這些說明會指導助理在每個新對話中的回覆方式。它們會隨你的第一則訊息一起送出，不會變更全域 AGENTS.md 規則。',