feat(chat): add export session as markdown workflow (#934)

Add a download button in the chat view that exports the current session's
messages as a formatted Markdown file. User and assistant turns are
separated by horizontal rules. The filename is derived from the session
title and today's date (e.g. fix-login-bug-2026-04-17.md).

packages/ui/src/lib/exportSession.ts

@@ -0,0 +1,58 @@
+import type { Message, Part } from '@opencode-ai/sdk/v2';
+
+type SessionMessageRecord = { info: Message; parts: Part[] };
+
+function extractTextFromParts(parts: Part[]): string {
+  return parts
+    .filter((p): p is Part & { type: 'text'; text: string } => p.type === 'text' && typeof p.text === 'string')
+    .map((p) => p.text)
+    .join('');
+}
+
+function formatMessageAsMarkdown(record: SessionMessageRecord): string {
+  const role = record.info.role === 'user' ? '### User' : '### Assistant';
+  const text = extractTextFromParts(record.parts).trim();
+
+  if (!text) return '';
+  return `${role}\n\n${text}`;
+}
+
+export function formatSessionAsMarkdown(
+  messages: SessionMessageRecord[],
+  sessionTitle?: string | null,
+): string {
+  const title = sessionTitle?.trim() || 'Session';
+  const date = new Date().toISOString().split('T')[0];
+
+  const header = `# ${title}\n\n*Exported on ${date}*\n\n---\n\n`;
+
+  const body = messages
+    .map(formatMessageAsMarkdown)
+    .filter(Boolean)
+    .join('\n\n---\n\n');
+
+  return header + body;
+}
+
+export function downloadAsMarkdown(content: string, filename: string): void {
+  const blob = new Blob([content], { type: 'text/markdown;charset=utf-8' });
+  const url = URL.createObjectURL(blob);
+  const anchor = document.createElement('a');
+  anchor.href = url;
+  anchor.download = filename;
+  document.body.appendChild(anchor);
+  anchor.click();
+  document.body.removeChild(anchor);
+  URL.revokeObjectURL(url);
+}
+
+export function buildExportFilename(sessionTitle?: string | null): string {
+  const base = sessionTitle?.trim() || 'session';
+  const safe = base
+    .toLowerCase()
+    .replace(/[^a-z0-9]+/g, '-')
+    .replace(/^-+|-+$/g, '')
+    .slice(0, 60);
+  const date = new Date().toISOString().split('T')[0];
+  return `${safe}-${date}.md`;
+}