Use GitHub-Flavored Markdown alert syntax for Note/Tip/Warning/Important blocks

Markdown export now emits > [!NOTE]/[!TIP]/[!WARNING]/[!IMPORTANT] for
text-block callouts, giving them the same colored/icon-labeled treatment
as the PDF/HTML/DOCX exports on renderers that support GFM alerts
(GitHub, Azure DevOps wikis, etc.), while degrading gracefully to a
plain blockquote elsewhere.

exporters/markdown.js

@@ -84,7 +84,11 @@ function exportMarkdown(ast, outDir, template = {}) {
 function emitBlocks(lines, step, position) {
   for (const tb of stepBlocks(step).filter((b) => b.kind === 'text' && b.position === position)) {
     const label = LEVEL_LABEL[tb.level] || 'Note';
-    lines.push(`> **${label}${tb.title ? `: ${tb.title}` : ''}**`);
+    // GitHub-Flavored Markdown alert syntax — renders with a colored,
+    // icon-labeled box on GitHub/Azure DevOps wikis and several other
+    // viewers; degrades to a plain blockquote elsewhere.
+    lines.push(`> [!${label.toUpperCase()}]`);
+    if (tb.title) lines.push(`> **${tb.title}**`);
     const body = htmlToMarkdown(tb.descriptionHtml);
     if (body) lines.push(`> ${body.replace(/\n/g, '\n> ')}`);
     lines.push('');

tests/unit/exporters-text.test.js

@@ -109,10 +109,11 @@ test('Markdown export: TOC anchors resolve, images exist, blocks rendered', (t)
   assert.equal(lines[fenceStart + 1], '0 2 * * * /usr/local/bin/acmesync --backup');
   assert.equal(lines[fenceStart + 2], '```');
   assert.ok(lines.some((l) => /^\| Day \| Window \|$/.test(l)), 'table header row');
-  // Warning text block became a blockquote with its content.
-  const warnIdx = lines.findIndex((l) => l.startsWith('> **Warning: Access**'));
+  // Warning text block became a GFM alert blockquote with its content.
+  const warnIdx = lines.findIndex((l) => l.startsWith('> [!WARNING]'));
   assert.ok(warnIdx > 0);
-  assert.equal(lines[warnIdx + 1], '> Admins only.');
+  assert.equal(lines[warnIdx + 1], '> **Access**');
+  assert.equal(lines[warnIdx + 2], '> Admins only.');
 });
 
 test('Confluence export writes storage-format XML and image attachments', (t) => {