Add a voice.llm_router flag, default off

Wires cmd/mavend/voice.go to build the LLM router when the operator asks
for it. Default false, so nothing changes on the deploy box.
Look at pickLLMRouter: the flag on with no llama-server logs one line and
keeps the classifier, it never fails a turn.
The default stays off until the router can refuse (#359) and the extractor
runs on LLM decisions — both noted as TODOs in config.go.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CGeSZxh1DCtRxmFVSYVGvJ

cmd/mavend/llmrouter_flag_test.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"testing"
+	"time"
+
+	"github.com/kami/maven/internal/llm"
+)
+
+func TestPickLLMRouterOff(t *testing.T) {
+	if r := pickLLMRouter(false, llm.New("http://127.0.0.1:1", time.Second)); r != nil {
+		t.Error("flag off should give no LLM router")
+	}
+}
+
+// The operator can turn the flag on without an LLM phraser configured. That must
+// leave the classifier running, not panic.
+func TestPickLLMRouterOnWithoutClient(t *testing.T) {
+	if r := pickLLMRouter(true, nil); r != nil {
+		t.Error("no llama-server should give no LLM router")
+	}
+}
+
+func TestPickLLMRouterOn(t *testing.T) {
+	if r := pickLLMRouter(true, llm.New("http://127.0.0.1:1", time.Second)); r == nil {
+		t.Error("flag on with a client should give an LLM router")
+	}
+}

cmd/mavend/voice.go

@@ -199,8 +199,6 @@ func wireVoice(cfg *config.Config, coreAPI ipc.CoreAPI, phr phraser.Phraser, mem
 	if lp, ok := phr.(*phraser.LLMPhraser); ok {
 		llmClient = llm.New(lp.BaseURL(), 60*time.Second)
 	}
-	// LLM router disabled — the classifier handles routing reliably.
-
 	// ----- router (the cascade; floor examples seed the classifier) -----
 	// The act matcher's allowlist is exactly the enabled tool names — the
 	// router only matches acts the executor can run (one source of truth).
@@ -208,7 +206,11 @@ func wireVoice(cfg *config.Config, coreAPI ipc.CoreAPI, phr phraser.Phraser, mem
 	if threshold <= 0 {
 		threshold = config.DefaultRouterThreshold
 	}
-	rtr := buildRouter(emb, matcher, threshold, nil) // LLM router disabled
+	// Both routing paths are weak on held-out utterances — the classifier gets
+	// 36.8% of intents right, the resident model 50.0% and much slower. Off by
+	// default (see config.VoiceConfig.LLMRouter); the classifier always stays
+	// wired as the fallback, so a model error never breaks a turn.
+	rtr := buildRouter(emb, matcher, threshold, pickLLMRouter(cfg.Voice.LLMRouter, llmClient))
 
 	// ----- sessions registry (shared with voicesink) -----
 	sessions := voice.NewSessions()
@@ -1048,6 +1050,21 @@ func (h *reactiveHandler) reply(ctx context.Context, text string, _ []string) (v
 	return voice.PushToTalkResp{ReplyText: text, ReplyAudio: audioOut}, nil
 }
 
+// pickLLMRouter returns the LLM router when the operator asked for it and there
+// is a llama-server to talk to, and nil otherwise. nil is safe: the cascade then
+// routes with the classifier, so an unusable setting costs accuracy, not turns.
+func pickLLMRouter(enabled bool, c *llm.Client) *router.LLMRouter {
+	if !enabled {
+		return nil
+	}
+	if c == nil {
+		log.Printf("voice: voice.llm_router is on but there is no llama-server to route with (the phraser is not an LLM phraser) — using the classifier instead")
+		return nil
+	}
+	log.Printf("voice: LLM router enabled")
+	return router.NewLLMRouter(c)
+}
+
 // buildRouter constructs the reactive-path router with the given embedder
 // and confidence threshold.
 //   - stage-0 grammars from DefaultActMatcher whose fn allowlist is exactly