feat: add Markdown transcription prompt for document formatting

src/prompts/md_transcript.md

@@ -0,0 +1,5 @@
+Provide a verbatim transcription of this document into Markdown.
+- Preserve all content exactly (no summarizing or rewriting)
+- Keep the original structure using Markdown headers
+- Preserve lists, emphasis, links, tables, code blocks
+- Output Markdown only