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/db.rs

@@ -606,8 +606,8 @@ impl Db {
             .await?;
         for pick in picks {
             sqlx::query(
-                "INSERT INTO issue_articles (issue_date, article_id, section, position, is_lead, summary, why)
-                 VALUES (?, ?, ?, ?, ?, ?, ?)",
+                "INSERT INTO issue_articles (issue_date, article_id, section, position, is_lead, summary)
+                 VALUES (?, ?, ?, ?, ?, ?)",
             )
             .bind(date.to_string())
             .bind(pick.article.id)
@@ -615,7 +615,6 @@ impl Db {
             .bind(pick.position)
             .bind(pick.is_lead)
             .bind(pick.summary.as_deref())
-            .bind(pick.why.as_deref())
             .execute(&mut *tx)
             .await?;
         }