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/db.rs

@@ -25,9 +25,9 @@ pub static MIGRATOR: sqlx::migrate::Migrator = sqlx::migrate!("./migrations");
 
 /// `kv` key holding the ingest watermark (§3.1).
 pub const KV_WATERMARK: &str = "ingest_watermark";
-/// `kv` key holding the current taste profile document (§3.6).
+/// `kv` key holding the current system-prompt profile document (§8.4).
 pub const KV_TASTE_PROFILE: &str = "taste_profile";
-/// `kv` key holding the taste profile version/build time (§3.6).
+/// `kv` key holding the profile version/build time (§8.2).
 pub const KV_PROFILE_VERSION: &str = "profile_version";
 
 #[derive(Debug, thiserror::Error)]
@@ -107,7 +107,7 @@ impl Db {
     }
 
     // -----------------------------------------------------------------
-    // kv (§3.1 watermark, §3.6 taste profile)
+    // kv (§3.1 watermark, §8 profile)
     // -----------------------------------------------------------------
 
     pub async fn kv_get(&self, key: &str) -> Result<Option<String>> {
@@ -635,7 +635,7 @@ impl Db {
     }
 
     // -----------------------------------------------------------------
-    // runs (§3.6 cost guardrail, §3.13)
+    // runs (§3.13, plan §7.6)
     // -----------------------------------------------------------------
 
     /// Insert a `running` row at the top of `generate`; returns `runs.id`.