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/core/src/store.test.ts

@@ -317,6 +317,21 @@ describe("TaskStore", () => {
 
   // ── Settings tests ────────────────────────────────────────────────
 
+  describe("model settings", () => {
+    it("persists defaultProvider and defaultModelId and returns them via getSettings", async () => {
+      await store.updateSettings({ defaultProvider: "anthropic", defaultModelId: "claude-sonnet-4-5" });
+      const settings = await store.getSettings();
+      expect(settings.defaultProvider).toBe("anthropic");
+      expect(settings.defaultModelId).toBe("claude-sonnet-4-5");
+    });
+
+    it("default settings do not include model fields", async () => {
+      const settings = await store.getSettings();
+      expect(settings.defaultProvider).toBeUndefined();
+      expect(settings.defaultModelId).toBeUndefined();
+    });
+  });
+
   describe("worktreeInitCommand setting", () => {
     it("persists worktreeInitCommand and returns it via getSettings", async () => {
       await store.updateSettings({ worktreeInitCommand: "pnpm install" });

packages/core/src/types.ts

@@ -99,6 +99,14 @@ export interface Settings {
    *  commit scope (e.g. `feat(HAI-001): ...`). When false, the scope is
    *  omitted (e.g. `feat: ...`). Default: true. */
   includeTaskIdInCommit?: boolean;
+  /** Default AI model provider name (e.g. `"anthropic"`, `"openai"`).
+   *  Must be set together with `defaultModelId`. When both are undefined,
+   *  the engine uses pi's automatic model resolution. */
+  defaultProvider?: string;
+  /** Default AI model ID within the provider (e.g. `"claude-sonnet-4-5"`).
+   *  Must be set together with `defaultProvider`. When both are undefined,
+   *  the engine uses pi's automatic model resolution. */
+  defaultModelId?: string;
 }
 
 export const DEFAULT_SETTINGS: Settings = {
@@ -111,6 +119,8 @@ export const DEFAULT_SETTINGS: Settings = {
   recycleWorktrees: false,
   taskPrefix: undefined,
   includeTaskIdInCommit: true,
+  defaultProvider: undefined,
+  defaultModelId: undefined,
 };
 
 export interface BoardConfig {