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/src/core/session-manager.ts

@@ -696,6 +696,19 @@ export class SessionManager {
 		return this.byId.get(id);
 	}
 
+	/**
+	 * Get all direct children of an entry.
+	 */
+	getChildren(parentId: string): SessionEntry[] {
+		const children: SessionEntry[] = [];
+		for (const entry of this.byId.values()) {
+			if (entry.parentId === parentId) {
+				children.push(entry);
+			}
+		}
+		return children;
+	}
+
 	/**
 	 * Get the label for an entry, if any.
 	 */