Drop the editor's "Why it's here" line

After a rewrite of the prompt rule and a few more live issues the line still
read as a second abstract, so it goes entirely: the rule and the "why" key in
the editor's JSON, the parsing and 14-word cap, the Pick field, the
issue_articles and candidate_runs writes, the Brief prompt's per-pick why,
the explain output, the dashboard "editor" columns, and the rendering on the
web index, article and public pages, the Atom feed and both EPUB pages.

The "Matches: …" interests line stays (restyled in the next commit). The
issue_articles.why and candidate_runs.editor_why columns remain in the
schema unused; stored issue snapshots that still carry "why" deserialize
because Pick ignores unknown fields.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ESJbbzQ1b8fs5AA65tKw2q

src/epub/templates/chapter.xhtml

@@ -10,11 +10,8 @@
 {% if understanding.kicker.is_some() || understanding.topics.is_some() %}
         <p class="rubric">{% if let Some(kicker) = understanding.kicker %}<span class="kicker">{{ kicker }}</span>{% if let Some(topics) = understanding.topics %} &#160; {{ topics }}{% endif %}{% else %}{% if let Some(topics) = understanding.topics %}{{ topics }}{% endif %}{% endif %}</p>
 {% endif %}
-{% if why.is_some() || !understanding.interests.is_empty() %}
+{% if !understanding.interests.is_empty() %}
         <div class="why">
-{% if let Some(text) = why %}
-          <p class="why-line"><em>Why it&#39;s here: {{ text }}</em></p>
-{% endif %}
 {% if !understanding.interests.is_empty() %}
           <p class="why-matches">Matches: {{ understanding.interests_line() }}</p>
 {% endif %}