On macOS, mimetypes.guess_type() may not recognize .md files, causing
stat to report application/octet-stream and breaking test assertions.
Explicitly map .md files to text/markdown so the behavior is stable
across platforms.
* docs: rename vault_dir to workspace_dir in documentation and examples
* refactor(extract): format long method call across multiple lines
* refactor(extract): format system prompt parameters for better readability
