feat(brain_answer): Qwen3-Reranker cross-encoder filter (opt-in)

Adds an opt-in cross-encoder rerank step between BM25 retrieval and LLM
synthesis. With BRAIN_RERANKER_URL set, brain_answer retrieves BM25
top-20, scores each excerpt against the query via Qwen3-Reranker on
Ollama, drops the "no" answers, and forwards up to 5 surviving sources
to the LLM. Unset, behaviour is unchanged (BM25 top-10 → LLM).

The reranker is a *filter*, not a re-ranker: Qwen3-Reranker emits a
binary yes/no token under its native chat template, and ties within the
"yes" set are broken by BM25 rank — what got retrieved first stays
ahead.

New package ingestion/internal/reranker:
- Client with URL, Model, HTTP fields.
- New(url, model) returns nil on empty url so callers can treat
  "feature disabled" as a single nil check.
- Score(ctx, query, docs) issues one /api/generate call per doc using
  the Qwen3-Reranker yes/no chat template (verbatim, because the model
  was trained on this exact wording). Parses the first non-think token.

Wiring:
- mcp.Server gains a WithReranker fluent setter to keep NewServer
  signature stable.
- brain_answer's BM25 limit jumps to 20 only when a reranker is wired,
  to give the filter something to do.
- cmd/server/main.go reads BRAIN_RERANKER_URL (+ optional
  BRAIN_RERANKER_MODEL, default dengcao/Qwen3-Reranker-0.6B:F16).

Tests cover: nil-on-empty-url, ordered yes/no scoring, request shape
(model, prompt contents, yes/no template), ambiguous response → 0,
empty doc slice, upstream-error propagation, plus an end-to-end
brain_answer integration that proves only the relevant note reaches the
LLM when noise.md is rejected.

Closes hyperguild#7.

ingestion/internal/reranker/reranker_test.go

@@ -0,0 +1,119 @@
+package reranker_test
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/reranker"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// fakeOllama responds to /api/generate based on a per-document
+// {needle → answer} map: if the prompt contains the needle, returns
+// the mapped answer.
+type fakeOllama struct {
+	t        *testing.T
+	answers  map[string]string // needle → "yes" or "no"
+	calls    int
+	lastBody map[string]any
+}
+
+func (f *fakeOllama) handler() http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		require.Equal(f.t, http.MethodPost, r.Method)
+		require.Equal(f.t, "/api/generate", r.URL.Path)
+		body, err := io.ReadAll(r.Body)
+		require.NoError(f.t, err)
+		var p map[string]any
+		require.NoError(f.t, json.Unmarshal(body, &p))
+		f.calls++
+		f.lastBody = p
+		prompt := p["prompt"].(string)
+		answer := "no"
+		for needle, a := range f.answers {
+			if strings.Contains(prompt, needle) {
+				answer = a
+				break
+			}
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"model": p["model"], "response": answer, "done": true,
+		})
+	})
+}
+
+func TestNew_EmptyURLReturnsNil(t *testing.T) {
+	assert.Nil(t, reranker.New("", "model"))
+}
+
+func TestScore_YesAndNoOrdered(t *testing.T) {
+	f := &fakeOllama{t: t, answers: map[string]string{
+		"alpha doc": "yes",
+		"beta doc":  "no",
+		"gamma doc": "yes",
+	}}
+	srv := httptest.NewServer(f.handler())
+	defer srv.Close()
+	c := reranker.New(srv.URL, "test-model")
+	require.NotNil(t, c)
+
+	scores, err := c.Score(context.Background(), "what is alpha",
+		[]string{"alpha doc body", "beta doc body", "gamma doc body"})
+	require.NoError(t, err)
+	require.Len(t, scores, 3)
+	assert.Equal(t, 1.0, scores[0])
+	assert.Equal(t, 0.0, scores[1])
+	assert.Equal(t, 1.0, scores[2])
+	assert.Equal(t, 3, f.calls)
+}
+
+func TestScore_SendsCorrectShape(t *testing.T) {
+	f := &fakeOllama{t: t, answers: map[string]string{"hello": "yes"}}
+	srv := httptest.NewServer(f.handler())
+	defer srv.Close()
+	c := reranker.New(srv.URL, "qwen3-rerank")
+	_, err := c.Score(context.Background(), "greeting", []string{"hello world"})
+	require.NoError(t, err)
+	assert.Equal(t, "qwen3-rerank", f.lastBody["model"])
+	prompt := f.lastBody["prompt"].(string)
+	assert.Contains(t, prompt, "greeting")
+	assert.Contains(t, prompt, "hello world")
+	assert.Contains(t, prompt, `"yes" or "no"`)
+}
+
+func TestScore_HandlesAmbiguousResponse(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_ = json.NewEncoder(w).Encode(map[string]any{"response": "maybe — unclear", "done": true})
+	}))
+	defer srv.Close()
+	c := reranker.New(srv.URL, "m")
+	scores, err := c.Score(context.Background(), "q", []string{"d"})
+	require.NoError(t, err)
+	// Anything that does not start with "yes" (case-insensitive, after
+	// whitespace/think trim) is treated as "no" = 0.
+	assert.Equal(t, []float64{0}, scores)
+}
+
+func TestScore_EmptyDocsReturnsEmpty(t *testing.T) {
+	c := reranker.New("http://127.0.0.1:1", "m")
+	scores, err := c.Score(context.Background(), "q", nil)
+	require.NoError(t, err)
+	assert.Empty(t, scores)
+}
+
+func TestScore_UpstreamErrorPropagates(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	defer srv.Close()
+	c := reranker.New(srv.URL, "m")
+	_, err := c.Score(context.Background(), "q", []string{"d"})
+	require.Error(t, err)
+}