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.ts

@@ -94,6 +94,11 @@ export interface AgentRuntime {
   readonly supportsToolExecution: boolean
   /** Whether request-scoped GoodBuddy data tools can reach this runtime. */
   readonly supportsScopedDataTools?: boolean
+  /**
+   * Whether `trustedInstructions` reach the model's system layer for every
+   * turn. Runtimes without this guarantee receive them inside the prompt.
+   */
+  readonly consumesTrustedInstructions?: boolean
   readonly capability?: 'chat' | 'image-generation'
   getStatus(): Promise<AgentRuntimeStatus>
   testConnection?(): Promise<AgentRuntimeStatus>