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/mcp/server.go

@@ -32,19 +32,21 @@ type rpcError struct {
 
 // Server handles MCP JSON-RPC over HTTP for the ingestion service.
 type Server struct {
-	brainDir string
-	pipeline pipeline.Config
-	llm      pipeline.CompleteFunc
+	brainDir  string
+	pipeline  pipeline.Config
+	llm       pipeline.CompleteFunc
+	answerLLM pipeline.CompleteFunc // nil = brain_answer and brain_classify unavailable
 }
 
 // NewServer constructs a Server bound to brainDir. pipelineCfg supplies the
 // LLM-backed pipeline; llm may be nil for non-LLM tools only.
-func NewServer(brainDir string, pipelineCfg *pipeline.Config, llm pipeline.CompleteFunc) *Server {
+// answerLLM drives brain_answer and brain_classify; nil disables those tools.
+func NewServer(brainDir string, pipelineCfg *pipeline.Config, llm pipeline.CompleteFunc, answerLLM pipeline.CompleteFunc) *Server {
 	cfg := pipeline.Config{}
 	if pipelineCfg != nil {
 		cfg = *pipelineCfg
 	}
-	return &Server{brainDir: brainDir, pipeline: cfg, llm: llm}
+	return &Server{brainDir: brainDir, pipeline: cfg, llm: llm, answerLLM: answerLLM}
 }
 
 func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
@@ -140,6 +142,10 @@ func (s *Server) handleCall(ctx context.Context, name string, args json.RawMessa
 		return s.brainIngest(ctx, args)
 	case "session_log":
 		return s.sessionLog(ctx, args)
+	case "brain_answer":
+		return s.brainAnswer(ctx, args)
+	case "brain_classify":
+		return s.brainClassify(ctx, args)
 	default:
 		return nil, fmt.Errorf("unknown tool: %s", name)
 	}