refactor: replaced ReturnType<typeof setTimeout> with Timer type

- Replaced usage of `ReturnType<typeof setTimeout>` and `ReturnType<typeof setInterval>` with the explicit `Timer` type across the codebase.
- Updated several type definitions and function signatures to use concrete types instead of inferred return types for improved clarity and maintainability.

packages/coding-agent/test/goals/goal-mode-integration.test.ts

@@ -1,5 +1,6 @@
 import { afterAll, afterEach, beforeAll, beforeEach, describe, expect, it, vi } from "bun:test";
 import * as path from "node:path";
+import type { Model } from "@oh-my-pi/pi-ai";
 import { Agent } from "@oh-my-pi/pi-agent-core";
 import { ModelRegistry } from "@oh-my-pi/pi-coding-agent/config/model-registry";
 import { resetSettingsForTest, Settings } from "@oh-my-pi/pi-coding-agent/config/settings";
@@ -41,7 +42,7 @@ type GoalHarness = {
 type SharedFixture = {
 	authStorage: AuthStorage;
 	modelRegistry: ModelRegistry;
-	model: NonNullable<ReturnType<ModelRegistry["find"]>>;
+	model: Model;
 	baseDir: TempDir;
 };