Curation v2 step 7: cleanup, prune paths, implementation notes

Dead code and stale v1 comments removed (clippy -W dead_code clean, the
three world.rs warnings fixed), the Brief chapter's TOC title renamed from
"From the Editor", features prune now also sweeps article_assessments and
generate runs the sweep once after publishing, the example config is
tested key-for-key against Config::default(), README commands match
--help, and docs/plans/2026-08-15-implementation-notes.md records the
Anthropic and Voyage facts, the new tables, the budget-day rule and the
lock.

Implemented by a Claude agent from docs/plans/curation-v2-briefs/step7.md.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A1rCLQeKBgnBo3oTgHuTMe

tests/e2e_pipeline.rs

@@ -596,7 +596,7 @@ async fn llm_pipeline_runs_against_a_mock_backend() {
         "every candidate came back assessed"
     );
 
-    let lineup = curator.select(candidates, date()).await.expect("stage B");
+    let lineup = curator.select(candidates, date()).await.expect("editor");
     assert_eq!(lineup.picks.len(), 5);
     assert_eq!(
         lineup.lead().map(|p| p.article.id),