[flthibaud]

The cleanup rule removed every element carrying tabindex="-1". Sites put that attribute on real content (GitHub on every heading, many themes on <main> or on the <div id="content"> a skip link points to), so headings or whole article bodies were dropped.

• The rule now removes only a <div tabindex="-1"> with no id, which is what the Le Monde promo blocks it was written for look like. A div with an id is a skip-link target and is kept.
• New test case: a heading, a <main> and a <div id="content"> with tabindex="-1" all survive. The existing case and the Le Monde golden file still pass.

Tested with go vet and go test -race ./internal/readability.

Closes #38

[flthibaud]

Review: OK to merge.

• First version limited the rule to <a> and <div>; sent back because <div id="content" tabindex="-1"> (a common skip-link target wrapping the article) would still be deleted. The rule is now: bare <div tabindex="-1"> without an id only.
• Residual risk, accepted: a content div with tabindex="-1" and no id is still removed. None found in the fixtures.
• gofmt, vet and go test -race ./internal/readability pass, golden files included.