fix(search): cast query embedding to ::vector in search paper (DQ-4)

`codex search paper` crashed on the live DB with "operator does not exist:
vector <-> double precision[]". The pgvector `<->` operator needs its RHS
typed as vector; peer call sites (mcp_server.py, wiki.py) cast but the CLI
did not. Mocked unit tests never exercise real pgvector, so it went unseen.

Add the cast to both `<->` occurrences and a regression guard asserting the
SQL contains `::vector`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

codex/cli.py

@@ -84,10 +84,10 @@ def search_paper(
         rows = conn.execute(
             """
             SELECT id, title, year,
-                   abstract_emb <-> %(emb)s AS distance
+                   abstract_emb <-> %(emb)s::vector AS distance
             FROM   papers
             WHERE  abstract_emb IS NOT NULL
-            ORDER  BY abstract_emb <-> %(emb)s
+            ORDER  BY abstract_emb <-> %(emb)s::vector
             LIMIT  %(limit)s
             """,
             {"emb": emb, "limit": limit},