Responsible note under the title; followup pages out of the nav until a chain exists

- The "Asked by X" line moves from the page footer to directly below
  the hero, where the question it belongs to is.
- New Question.followup flag: a post-submission page only appears in
  the question nav once the visitor's context actually carries an
  answer chain - nobody qualifies for "what happens now" before
  something happened.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

style/main.css

@@ -274,6 +274,7 @@ main.not-found {
   color: var(--ink-dim);
   font-size: 0.85rem;
   line-height: 1.6;
+  margin: 0.6rem 1rem 1.8rem;
 }
 
 .question-responsible a {