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/dashboard/run.html

@@ -59,7 +59,7 @@
 </form>
 {% include "dashboard/_pager.html" %}
 {% if candidates.len() > 1 %}<input type="search" class="table-filter" placeholder="Filter rows on this page" aria-label="Filter rows on this page" data-table-filter>{% endif %}<div class="scroll-x tall"><table class="candidates" data-filter>
-<thead><tr><th>title</th><th>feed</th><th class="num">words</th><th>stage</th><th>reason</th><th>admitted by</th><th class="num">utility</th><th class="num">rank</th><th class="num">cluster</th><th class="num">triage</th><th class="num">quality</th><th class="num">fit</th><th>flags</th><th>editor</th></tr></thead>
-<tbody>{% for candidate in candidates %}{% include "_candidate_row.html" %}{% endfor %}{% if candidates.is_empty() %}<tr><td colspan="14" class="text-muted">No candidates match this filter.</td></tr>{% endif %}</tbody></table></div>
+<thead><tr><th>title</th><th>feed</th><th class="num">words</th><th>stage</th><th>reason</th><th>admitted by</th><th class="num">utility</th><th class="num">rank</th><th class="num">cluster</th><th class="num">triage</th><th class="num">quality</th><th class="num">fit</th><th>flags</th></tr></thead>
+<tbody>{% for candidate in candidates %}{% include "_candidate_row.html" %}{% endfor %}{% if candidates.is_empty() %}<tr><td colspan="13" class="text-muted">No candidates match this filter.</td></tr>{% endif %}</tbody></table></div>
 {% include "dashboard/_pager.html" %}
 </section>{% endblock %}