fix(brain_answer): reranker is a filter, not a gate — fall back to BM25 when it keeps nothing

The Qwen3-Reranker is a web-search cross-encoder. Against conversational /
personal-intent queries (e.g. 'what am I optimizing toward?') it scores every
candidate as 'no', so brain_answer collapsed to 'No relevant content found'
even though BM25 had retrieved on-topic notes (incl. the telos wing). Treat
the reranker as a filter: when it keeps zero results, fall back to the
BM25/vector ordering (capped to the no-reranker depth of 10). Refs brain#11.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ingestion/internal/mcp/tools_answer.go

@@ -77,9 +77,22 @@ func (s *Server) brainAnswer(ctx context.Context, args json.RawMessage) (json.Ra
 		return nil, fmt.Errorf("search: %w", err)
 	}
 	if s.reranker != nil && len(results) > 0 {
-		results, err = rerankResults(ctx, s.reranker, a.Query, results, 5)
-		if err != nil {
-			return nil, fmt.Errorf("rerank: %w", err)
+		reranked, rerr := rerankResults(ctx, s.reranker, a.Query, results, 5)
+		if rerr != nil {
+			return nil, fmt.Errorf("rerank: %w", rerr)
+		}
+		// The reranker is a filter, not a gate. The Qwen3-Reranker is a
+		// web-search cross-encoder: against a conversational / personal-
+		// intent query ("what am I optimizing toward?") it scores even
+		// on-topic notes as "no", which would collapse the whole answer to
+		// "no relevant content" despite BM25 having retrieved relevant
+		// content. When the reranker keeps nothing, fall back to the
+		// BM25/vector ordering (capped to the no-reranker depth) rather
+		// than returning an empty answer.
+		if len(reranked) > 0 {
+			results = reranked
+		} else if len(results) > 10 {
+			results = results[:10]
 		}
 	}
 	if len(results) == 0 {