feat(HAI-098): add model selection and configuration support

- Extend Settings type with model provider and model name fields
- Add backend API endpoint to list available models
- Add frontend API client for fetching models
- Add model settings section to SettingsModal with provider/model dropdowns
- Update engine executor to use the selected model from settings

packages/engine/src/executor.ts

@@ -349,6 +349,8 @@ export class TaskExecutor {
           customTools,
           onText: agentLogger.onText,
           onToolStart: agentLogger.onToolStart,
+          defaultProvider: settings.defaultProvider,
+          defaultModelId: settings.defaultModelId,
         });
 
         // Register session so the pause listener can terminate it

packages/engine/src/pi.ts

@@ -30,6 +30,10 @@ export interface AgentOptions {
   onText?: (delta: string) => void;
   onToolStart?: (name: string, args?: Record<string, unknown>) => void;
   onToolEnd?: (name: string, isError: boolean) => void;
+  /** Default model provider (e.g. "anthropic"). Used with `defaultModelId` to select a specific model. */
+  defaultProvider?: string;
+  /** Default model ID within the provider (e.g. "claude-sonnet-4-5"). Used with `defaultProvider`. */
+  defaultModelId?: string;
 }
 
 /**
@@ -50,6 +54,11 @@ export async function createHaiAgent(options: AgentOptions): Promise<AgentResult
     retry: { enabled: true, maxRetries: 3 },
   });
 
+  // Resolve explicit model selection if provider and model ID are specified
+  const selectedModel = options.defaultProvider && options.defaultModelId
+    ? modelRegistry.find(options.defaultProvider, options.defaultModelId)
+    : undefined;
+
   const resourceLoader = new DefaultResourceLoader({
     cwd: options.cwd,
     settingsManager,
@@ -67,6 +76,7 @@ export async function createHaiAgent(options: AgentOptions): Promise<AgentResult
     customTools: options.customTools,
     sessionManager: SessionManager.inMemory(),
     settingsManager,
+    ...(selectedModel ? { model: selectedModel } : {}),
   });
 
   // Wire up event listeners

packages/engine/src/triage.ts

@@ -270,6 +270,8 @@ export class TriageProcessor {
           customTools: this.createTriageTools(),
           onText: agentLogger.onText,
           onToolStart: agentLogger.onToolStart,
+          defaultProvider: settings.defaultProvider,
+          defaultModelId: settings.defaultModelId,
         });
 
         try {