[YayoiNanoka]

The desktop app shell directly contains session mode commands, optimistic settings presentation, composer surface ownership checks, and parent-session breadcrumb assembly. Keeping these responsibilities inline makes the multi-window change in #6093 harder to review independently.

This refactor moves those existing rules into their owning conversation, session settings, and session navigation features. The shell continues to supply the same selection refs and commands. The navigation regression test now exercises the breadcrumb action, and the renderer architecture ledger is regenerated.

User-visible behavior stays unchanged. This provides a small, independently reviewable prerequisite for simplifying #6093; it introduces no window coordination or new persistence.

Validation: full build and workspace typecheck; all 3,847 desktop tests; strict renderer architecture check against face9409; app-shell hook check; lint, formatting, and Astryx surface inventory.

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桌面 app shell 直接包含 session 模式命令、乐观设置展示、输入框所属页面判断以及父 session 面包屑组装。这些职责放在 shell 内，使 #6093 的多窗口修改难以独立审查。

本次重构将已有规则移到对应的对话、session 设置和 session 导航模块。Shell 仍然传入相同的选择引用与命令。导航回归测试改为验证面包屑动作，并重新生成 renderer 架构清单。

用户可见行为保持不变。该 PR 为精简 #6093 提供一个小型、可独立审查的前置重构，不引入窗口协调或新的持久化。

验证：完整构建、全工作区类型检查、3,847 项桌面测试、以 face9409 为基线的严格架构检查，以及 app-shell hook、lint、格式和 Astryx 清单检查。