Fixes TypeScript build errors in CI where Tab objects were created
without the required originalMarkdown field.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Addresses issue #13 follow-up: editing a file no longer causes heavy
reformatting. Changes:

- Code blocks without language no longer get `plaintext` tag added
- Tables preserve compact pipe format (no extra spaces)
- Headings no longer insert extra blank lines before following content
- Consecutive text lines grouped as single paragraph with hard breaks
  instead of being split into separate paragraphs with blank lines
- Lists followed by paragraphs preserve blank line separation
- Code block content trimmed to avoid extra trailing blank line

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

…nversion"

This reverts commit 565784e.

Change heading output in htmlToMarkdown from \n\n suffix to \n,
since headings are self-terminating blocks in markdown.
Also consume trailing newline from HTML with \n? in regex.

fix: minimize markdown formatting changes during roundtrip