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/model-runtime.ts

@@ -1637,6 +1637,7 @@ async function* readAnthropicToolStream(
 export class ModelAgentRuntime implements AgentRuntime {
   readonly runtimeId = 'model'
   readonly requiresToolApproval = false
+  readonly consumesTrustedInstructions = true
   private readonly conversations = new Map<string, ConversationMessage[]>()
   private readonly conversationSummaries = new Map<
     string,