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

src/curate/llm.rs

@@ -122,7 +122,8 @@ pub struct UsageMeter {
 }
 
 impl UsageMeter {
-    /// Compatibility constructor for the existing DeepSeek call sites.
+    /// A meter priced from the `[deepseek]` table; the other providers build
+    /// theirs with [`UsageMeter::with_prices`].
     pub fn new(cfg: &DeepseekConfig, limit_usd: f64) -> Self {
         Self::with_prices(PriceTable::deepseek(cfg), limit_usd)
     }