docs: clarify sparse-not-wired and cite-boost tie-breaker (audit C-14, U-2)

- C-14: embed.py header now states that only the dense path is wired (search
  'hybrid' is dense + Postgres FTS); encode_sparse/encode are reserved for a
  future sparse-retrieval layer, not yet consumed.
- U-2: --cite-boost help clarifies it re-ranks within the top results (a
  tie-breaker), not a hard re-ranking — matching the small alpha effect.

Accepted (documented heuristics, no change): R-3 conflict detection is a
keyword-only signal labelled as such; R-12 classify_section is mostly 'body'
because chunks are word-windows that rarely start at a section header.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

codex/cli.py

@@ -102,7 +102,8 @@ def search_paper(
     cite_boost: bool = typer.Option(
         False,
         "--cite-boost",
-        help="Weight results by citation PageRank (F-15). Graceful when corpus < 5 papers.",
+        help="Re-rank the top results by citation PageRank — a within-page "
+        "tie-breaker, not a hard re-ranking (F-15). Graceful when corpus < 5 papers.",
     ),
 ) -> None:
     """Semantic similarity search over paper abstracts."""