[建议 / Feature] Plan accepted → auto-switch to a designated implementation model / 计划通过后自动切换到指定执行模型

提交前确认 · Pre-submission checklist

• 我已搜索过现有 issue，确认这不是重复提议 / I searched existing issues and confirmed this isn't a duplicate.
• 我已阅读 CONTRIBUTING.md / I've read CONTRIBUTING.md.

问题类别 · Category

模型设置 / 切换 · Model config / switching

涉及的 Agent 框架 · Agent framework

ZCode Agent（自研）

使用场景 · Use case

EN: When I plan with a strong model (e.g., GLM-5.3) and accept the plan, the same (expensive) model continues into implementation. To run the execution phase on a cheaper/faster model (e.g., GLM-5.3-Flash) I have to remember to switch manually via /model right after approving — if I forget, the whole implementation runs on the planning model. There is no automation path today: hooks only support seven events (none for plan acceptance) and no hook output can change the session model.

中文： 用强模型（如 GLM-5.3）完成计划并通过后，默认仍是同一个（贵的）模型继续实现。想改用更快/更便宜的模型（如 GLM-5.3-Flash）执行，必须在批准计划后手动 /model 切换；一旦忘记，整个实现阶段都跑在规划模型上。目前没有自动化途径：hooks 只支持七个事件（没有“计划通过”事件），也没有任何 hook 输出能改变会话模型。

建议方案 · Proposal

EN: Add an optional “implementation model” setting: when a plan is accepted (ExitPlanMode approved), the session automatically switches to the configured model for subsequent turns. Possible shapes:

• A per-session / per-workspace “plan execution model” setting
• A prompt at approval time (“Accept and continue with Flash?”)
• Optionally, a planning-model override so plan mode always runs the strong model

中文： 增加「计划执行模型」设置：当计划被接受（ExitPlanMode 批准）时，会话自动切换到配置的模型继续后续回合。可以做成：

• 每会话/工作区级的「计划执行模型」设置
• 批准计划时弹出选择（“接受并用 Flash 继续？”）
• 也可顺带支持规划模型覆盖，让计划模式始终用强模型

预期价值 · Expected value

EN: Planning benefits from the strongest model; routine implementation often doesn't. Automatic handoff = lower cost, faster turns, and removes the forgot-to-switch failure mode. Complements #482 (advisor-on-demand) and #392 (per-subagent models) by covering the plan→execute handoff on the session spine.

中文： 规划需要最强模型，常规实现未必。自动接力 = 更低成本、更快回合，并消除“忘记切换”的失败模式。与 #482（按需顾问）和 #392（子代理模型选择）互补，覆盖会话主线的计划→执行交接。

你认为的优先级 · Your perceived priority

中 · Medium

你使用的 ZCode 版本 / 环境 · ZCode version / environment

v0.16.5 / Windows desktop

补充材料 · Additional context

Related: #482 (dual-model advisor mode, open), #392 (subagent model/effort override, open), #177 (per-subagent model selection, completed), #619 (automatic task-aware model selection — related family, but different: not tied to plan acceptance / the plan→execute handoff). Today's hook events — SessionStart, UserPromptSubmit, PreToolUse, PermissionRequest, PostToolUse, PostToolUseFailure, Stop — include no plan-acceptance event and cannot switch the model.

Start by tracing ExitPlanMode approval and the /model model-switching path, then compare the listed hook event handling. Done means an optional session or workspace implementation-model setting uses the configured model after approval; the final setting scope and approval interaction must be resolved.