feat: add Codex session management and automatic restoration

tests/codexSessionState.test.ts

@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'bun:test';
+import {
+  codexSessionPageCounts,
+  retainVisibleCodexSessionIds,
+} from '../src/services/codexSessionState';
+
+describe('Codex session page state', () => {
+  it('counts active and archived sessions on the current page', () => {
+    expect(codexSessionPageCounts([
+      { archived: false },
+      { archived: true },
+      { archived: false },
+    ])).toEqual({ active: 2, archived: 1 });
+  });
+
+  it('drops selections that are no longer on the visible page', () => {
+    expect(Array.from(retainVisibleCodexSessionIds(
+      new Set(['thread-a', 'thread-c']),
+      [{ id: 'thread-a' }, { id: 'thread-b' }],
+    ))).toEqual(['thread-a']);
+  });
+});