fix: default search_docs to dense; rerank opt-in

Post-prefix eval (22 queries, 2026-09-30): dense MRR 0.966 vs
bm25+rerank 0.827. Watchtower keeps live RERANK_URL, so rerank
is gated on RERANK_ENABLED (default off) rather than URL presence.

deploy/docker-compose.yml

@@ -32,16 +32,16 @@ services:
       # add that hostname here. "*" disables the rebind check entirely.
       MCP_ALLOWED_HOSTS: "hvm-docs-mcp,localhost,127.0.0.1"
 
-      # Phase 6 — reranker sidecar (jina-reranker-v2-base via llama.cpp).
+      # Phase 6 — reranker sidecar is wired but off. Eval 2026-09-30
+      # (post nomic prefixes): dense MRR=0.966 vs bm25+rerank 0.827.
+      # Set RERANK_ENABLED=true to turn the sidecar back on.
       RERANK_URL: http://hvm-rerank:8080
       RERANK_POOL: "200"
       RERANK_TIMEOUT: "30"
+      RERANK_ENABLED: "false"
 
-      # Phase 8 — hybrid retrieval (BM25 + dense + RRF).
-      # Eval on the HVM corpus (eval/results/baseline.md, 2026-05-22) shows
-      # BM25-default + reranker beats hybrid on every metric (MRR 0.920 vs
-      # 0.875). Leaving HYBRID_SEARCH off so search_docs runs BM25-first +
-      # reranker; dense is the fallback when BM25 finds nothing.
+      # Phase 8 — hybrid retrieval (BM25 + dense + RRF). Off: dense is
+      # the default (eval/results/post-prefix.md). BM25 is fallback only.
       HYBRID_SEARCH: "false"
 
       # Phase 10 — usage telemetry.