feat(brain): add brain_answer and brain_classify MCP tools

Adds two new LLM-backed MCP tools to the ingestion service:

- brain_answer(query): BM25 retrieval + LLM synthesis → answer + sources
- brain_classify(text): classifies doc into type/title/tags via LLM

Adds llm.Router for primary→fallback routing (berget.ai → iguana).
Wired via BRAIN_LLM_PRIMARY_URL/BRAIN_LLM_FALLBACK_URL env vars;
no-op when unset so existing deployments are unaffected.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ingestion/internal/llm/router.go

@@ -0,0 +1,29 @@
+package llm
+
+import (
+	"context"
+	"fmt"
+)
+
+// Router calls Primary first; on any error falls back to Fallback.
+// Fallback may be nil, in which case primary errors are returned directly.
+type Router struct {
+	Primary  *Client
+	Fallback *Client
+}
+
+// Complete implements pipeline.CompleteFunc, routing through Primary then Fallback.
+func (r *Router) Complete(ctx context.Context, system, user string) (string, error) {
+	out, err := r.Primary.Complete(ctx, system, user)
+	if err == nil {
+		return out, nil
+	}
+	if r.Fallback == nil {
+		return "", fmt.Errorf("primary llm: %w", err)
+	}
+	out, err2 := r.Fallback.Complete(ctx, system, user)
+	if err2 != nil {
+		return "", fmt.Errorf("primary llm: %w; fallback llm: %v", err, err2)
+	}
+	return out, nil
+}

ingestion/internal/llm/router_test.go

@@ -0,0 +1,71 @@
+package llm
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRouter_PrimarySucceeds(t *testing.T) {
+	primary := mockServer(t, "from-primary")
+	defer primary.Close()
+	fallback := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Error("fallback must not be called when primary succeeds")
+	}))
+	defer fallback.Close()
+
+	r := &Router{
+		Primary:  New(primary.URL, "", "m", time.Second),
+		Fallback: New(fallback.URL, "", "m", time.Second),
+	}
+	out, err := r.Complete(context.Background(), "sys", "user")
+	require.NoError(t, err)
+	assert.Equal(t, "from-primary", out)
+}
+
+func TestRouter_FallsBackOnPrimaryError(t *testing.T) {
+	primary := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "unavailable", http.StatusServiceUnavailable)
+	}))
+	defer primary.Close()
+	fallback := mockServer(t, "from-fallback")
+	defer fallback.Close()
+
+	r := &Router{
+		Primary:  New(primary.URL, "", "m", time.Second),
+		Fallback: New(fallback.URL, "", "m", time.Second),
+	}
+	out, err := r.Complete(context.Background(), "sys", "user")
+	require.NoError(t, err)
+	assert.Equal(t, "from-fallback", out)
+}
+
+func TestRouter_BothFail(t *testing.T) {
+	fail := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "err", http.StatusBadGateway)
+	}))
+	defer fail.Close()
+
+	r := &Router{
+		Primary:  New(fail.URL, "", "m", time.Second),
+		Fallback: New(fail.URL, "", "m", time.Second),
+	}
+	_, err := r.Complete(context.Background(), "sys", "user")
+	assert.Error(t, err)
+}
+
+func TestRouter_NilFallback(t *testing.T) {
+	fail := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "err", http.StatusBadGateway)
+	}))
+	defer fail.Close()
+
+	r := &Router{Primary: New(fail.URL, "", "m", time.Second)}
+	_, err := r.Complete(context.Background(), "sys", "user")
+	assert.Error(t, err)
+}