Add Markdown export for tweet threads

[Redwallen]

What changed

• add twitter tweet --markdown to export a tweet thread into the current folder
• include the original tweet, numbered replies with author names, and linked images
• link quoted tweets instead of embedding their content
• name the file from the tweet's first sentence
• document the new option and add regression coverage

Why

Users can now save a complete tweet conversation as a readable Markdown document without downloading image files.

Validation

• pytest -q: 249 passed, 6 deselected
• ruff check .: passed
• mypy twitter_cli: passed