Add tree navigation tests and shared test utilities

- Add test/utilities.ts with shared helpers (API_KEY, userMsg, assistantMsg, createTestSession)
- Add agent-session-tree-navigation.test.ts with e2e tests for tree navigation
- Add getChildren() method to SessionManager
- Add summaryEntry to navigateTree return type
- Update existing tests to use shared utilities

packages/coding-agent/test/session-manager/tree-traversal.test.ts

@@ -1,29 +1,6 @@
 import { describe, expect, it } from "vitest";
 import { type CustomEntry, SessionManager } from "../../src/core/session-manager.js";
-
-function userMsg(text: string) {
-	return { role: "user" as const, content: text, timestamp: Date.now() };
-}
-
-function assistantMsg(text: string) {
-	return {
-		role: "assistant" as const,
-		content: [{ type: "text" as const, text }],
-		api: "anthropic-messages" as const,
-		provider: "anthropic",
-		model: "test",
-		usage: {
-			input: 1,
-			output: 1,
-			cacheRead: 0,
-			cacheWrite: 0,
-			totalTokens: 2,
-			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
-		},
-		stopReason: "stop" as const,
-		timestamp: Date.now(),
-	};
-}
+import { assistantMsg, userMsg } from "../utilities.js";
 
 describe("SessionManager append and tree traversal", () => {
 	describe("append operations", () => {