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/web/templates/_candidate_row.html

@@ -12,5 +12,4 @@
 <td class="num">{{ candidate.quality }}</td>
 <td class="num">{{ candidate.fit }}</td>
 <td class="cell-tight">{% if candidate.exploration %}<span class="badge">exploration</span>{% endif %}{% if candidate.auto_include %}<span class="badge">auto</span>{% endif %}</td>
-<td class="cell-wrap text-muted">{% if let Some(why) = candidate.editor_why %}{{ why }}{% endif %}</td>
 </tr>