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/agent-session-compaction.test.ts

@@ -19,8 +19,7 @@ import { ModelRegistry } from "../src/core/model-registry.js";
 import { SessionManager } from "../src/core/session-manager.js";
 import { SettingsManager } from "../src/core/settings-manager.js";
 import { codingTools } from "../src/core/tools/index.js";
-
-const API_KEY = process.env.ANTHROPIC_OAUTH_TOKEN || process.env.ANTHROPIC_API_KEY;
+import { API_KEY } from "./utilities.js";
 
 describe.skipIf(!API_KEY)("AgentSession compaction e2e", () => {
 	let session: AgentSession;