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

tests/e2e_pipeline.rs

@@ -619,11 +619,6 @@ async fn llm_pipeline_runs_against_a_mock_backend() {
         "the model's summaries were used, not excerpts"
     );
     assert!(editorial_doc.front_page_html.contains("storage internals"));
-    assert!(
-        lineup.picks.iter().all(|p| p.why.is_none()),
-        "the scripted editor gave no why lines"
-    );
-
     // Every scripted response was consumed, and the meter priced them (§3.6).
     assert_eq!(backend.calls(), 1 + 1 + 5 + 1);
     let total = meter.total();