feat: add markdown composer backend foundation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/shared/markdownText.test.ts

@@ -0,0 +1,21 @@
+import { describe, expect, test } from 'bun:test';
+
+import { buildMarkdownExcerpt, extractPlainTextFromMarkdown } from '@shared/utils/markdownText';
+
+describe('markdown text helpers', () => {
+  test('extracts readable text from common markdown syntax', () => {
+    expect(
+      extractPlainTextFromMarkdown('# Title\n\n**Bold** [Docs](https://example.com) and `code`.'),
+    ).toBe('Title Bold Docs and code.');
+  });
+
+  test('turns fenced code blocks into readable excerpts', () => {
+    expect(buildMarkdownExcerpt('```ts\nconst answer = 42;\n```', 48)).toBe('const answer = 42;');
+  });
+
+  test('truncates after stripping markdown syntax', () => {
+    expect(buildMarkdownExcerpt('**Alpha** beta gamma delta epsilon zeta eta theta', 18)).toBe(
+      'Alpha beta gamma d…',
+    );
+  });
+});