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/README.md

@@ -10,6 +10,7 @@ Provider 要求的附加元数据；GoodBuddy 只在直连链路或 Runtime 明
 | --- | --- |
 | [UI 设计](./ui-design.md) | 编辑入口、校验反馈、兼容性说明和无障碍行为 |
 | [技术设计](./technical-design.md) | 契约、持久化、合并优先级、Runtime 支持矩阵和验证 |
+| [执行方式与模型选择](./runtime-selection-design.md) | 会话/项目/全局分层解析、选择器行为、存储迁移、全局 `provider` 移除计划和用途模型规划 |
 | [功能进度](./progress.md) | 已验证实现、剩余工作和验证证据 |
 
 模型连接选择准确性也在本目录维护：技术设计第 8 节定义通道直连回退修复，