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.

src/main/agent/runtime-controller.ts

@@ -112,6 +112,10 @@ export class AgentRuntimeController implements AgentRuntime {
     return this.current.runtime.supportsScopedDataTools !== false
   }
 
+  get consumesTrustedInstructions(): boolean {
+    return this.current.runtime.consumesTrustedInstructions === true
+  }
+
   get capability(): AgentRuntime['capability'] {
     return this.current.runtime.capability
   }