Refresh AI region list after editor content loads

- Extract AI region scanning into a reusable utility with unit coverage.

- Refresh AI region dropdown state after drafts, reports, default templates, and selected templates write HTML into the editor.

- Keep the existing MutationObserver path for later DOM edits and inserted AI regions.

- Add E2E coverage for existing template AI regions appearing on initial report editor load.

- Update README, AGENTS, report editor, progress, and testing docs for AI region synchronization behavior.

e2e/report-ai-region.spec.ts

@@ -1,6 +1,19 @@
 import { expect, test } from '@playwright/test';
 import { loginByApi, uniqueId } from './helpers';
 
+test('existing template AI region appears in the AI writing target dropdown on page load', async ({ page }) => {
+  await loginByApi(page, '0001');
+
+  await page.goto('/report-editor');
+  await page.getByRole('button', { name: 'AI撰写' }).click();
+
+  const aiRegionSelect = page.locator('select').filter({
+    has: page.locator('option', { hasText: '手术步骤、术中出现的情况及处理' }),
+  });
+  await expect(aiRegionSelect).toHaveValue('手术步骤');
+  await expect(page.locator('select option', { hasText: '无可用 AI 区域' })).toHaveCount(0);
+});
+
 test('newly inserted AI region appears in the AI writing target dropdown immediately', async ({ page }) => {
   await loginByApi(page, '0001');
   const regionName = `AI区域${uniqueId('region')}`;