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_test.go

@@ -98,6 +98,42 @@ func TestBrainAnswer_RerankerFiltersBeforeLLM(t *testing.T) {
 	assert.NotContains(t, sawSources, "noise.md")
 }
 
+func TestBrainAnswer_RerankerKeepsNone_FallsBackToBM25(t *testing.T) {
+	brainDir := brainDirWithContent(t) // test.md BM25-matches "pass-rate logging"
+
+	// Reranker rejects every candidate ("no" to all) — models a
+	// web-search cross-encoder facing a conversational / personal-intent
+	// query, which is exactly when it wrongly scores on-topic notes as
+	// irrelevant. The answer must still synthesize from the BM25 hits, not
+	// collapse to "no relevant content".
+	rrSrv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_ = json.NewEncoder(w).Encode(map[string]any{"response": "no", "done": true})
+	}))
+	defer rrSrv.Close()
+
+	var sawSources string
+	llm := func(_ context.Context, _, user string) (string, error) {
+		sawSources = user
+		return "fallback answer", nil
+	}
+
+	srv := mcp.NewServer(brainDir, nil, nil, llm).
+		WithReranker(reranker.New(rrSrv.URL, "qwen3"))
+	ts := httptest.NewServer(srv)
+	defer ts.Close()
+
+	rpc := callTool(t, ts, "brain_answer", map[string]any{"query": "pass-rate logging"})
+	require.Nil(t, rpc["error"])
+
+	content := rpc["result"].(map[string]any)["content"].([]any)[0].(map[string]any)["text"].(string)
+	var result map[string]any
+	require.NoError(t, json.Unmarshal([]byte(content), &result))
+
+	assert.Equal(t, "fallback answer", result["answer"])
+	assert.NotEmpty(t, result["sources"], "reranker keeping nothing must fall back to BM25, not empty")
+	assert.Contains(t, sawSources, "test.md")
+}
+
 func TestBrainAnswer_NoLLM(t *testing.T) {
 	srv := mcp.NewServer(t.TempDir(), nil, nil, nil)
 	ts := httptest.NewServer(srv)