feat: layer runtime and model selection across conversation, project and global

- Store runtime selection as { provider?, model? } layers and resolve them per
  field (conversation -> project -> global); drop the legacy auto provider.
- Migrate stored selections (database v49) and clear deleted profile ids.
- Re-resolve selections in Main per request and at queue dispatch.
- Add ProjectRuntimeSelector and a two-column RuntimeModelPicker; manage
  link opens model connections; restore per-runtime default model tabs.
- Stop duplicating trusted instructions and history across OpenCode,
  Continue and DeepSeek Harness runtimes.
- Document the layered design, global provider removal plan and planned
  purpose-specific models.

docs/features/model-connections/ui-design.md

@@ -52,10 +52,9 @@ OpenCode 与 DeepSeek Harness 不使用该值。
 
 ## 会话与项目 Runtime 选择
 
-会话与项目选择器只提供直连模型和各 Runtime 入口，不再枚举每个 Runtime 与每个模型的
-组合；Runtime 模型在系统设置中配置。已保存的固定 Runtime 模型选择继续保留，项目
-选择器的选项与帮助文字均显示其固定模型，不能误报为跟随全局设置；用户可选择普通
-Runtime 入口恢复跟随。该展示规则不改变模型解析、远程 Agent 协议或执行权限。
+项目设置分“执行方式”和“模型”两个字段，输入框使用两栏选择器，全局默认模型仍在系统
+设置中按 Runtime 页签配置。分层规则、选择器状态、键盘行为和失效提示由
+[执行方式与模型选择设计](./runtime-selection-design.md#3-各入口行为已实现)唯一维护。
 
 ## Harness 模型来源展示