refactor: rewrite handler-base as Go module

Replace Python handler-base library with Go module providing:
- config: environment-based configuration
- health: HTTP health/readiness server for k8s probes
- natsutil: NATS/JetStream client with msgpack serialization
- telemetry: OpenTelemetry tracing and metrics setup
- clients: HTTP clients for LLM, embeddings, reranker, STT, TTS
- handler: base Handler runner wiring NATS + health + telemetry

Implements ADR-0061 Phase 1.

health/health_test.go

@@ -0,0 +1,77 @@
+package health
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"testing"
+	"time"
+)
+
+func TestHealthEndpoint(t *testing.T) {
+	srv := New(18080, "/health", "/ready", nil)
+	srv.Start()
+	defer srv.Stop(context.Background())
+	time.Sleep(50 * time.Millisecond)
+
+	resp, err := http.Get("http://localhost:18080/health")
+	if err != nil {
+		t.Fatalf("health request failed: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != 200 {
+		t.Errorf("expected 200, got %d", resp.StatusCode)
+	}
+	body, _ := io.ReadAll(resp.Body)
+	var data map[string]string
+	_ = json.Unmarshal(body, &data)
+	if data["status"] != "healthy" {
+		t.Errorf("expected status 'healthy', got %q", data["status"])
+	}
+}
+
+func TestReadyEndpointDefault(t *testing.T) {
+	srv := New(18081, "/health", "/ready", nil)
+	srv.Start()
+	defer srv.Stop(context.Background())
+	time.Sleep(50 * time.Millisecond)
+
+	resp, err := http.Get("http://localhost:18081/ready")
+	if err != nil {
+		t.Fatalf("ready request failed: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != 200 {
+		t.Errorf("expected 200, got %d", resp.StatusCode)
+	}
+}
+
+func TestReadyEndpointNotReady(t *testing.T) {
+	ready := false
+	srv := New(18082, "/health", "/ready", func() bool { return ready })
+	srv.Start()
+	defer srv.Stop(context.Background())
+	time.Sleep(50 * time.Millisecond)
+
+	resp, err := http.Get("http://localhost:18082/ready")
+	if err != nil {
+		t.Fatalf("ready request failed: %v", err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != 503 {
+		t.Errorf("expected 503 when not ready, got %d", resp.StatusCode)
+	}
+
+	ready = true
+	resp2, err := http.Get("http://localhost:18082/ready")
+	if err != nil {
+		t.Fatalf("ready request failed: %v", err)
+	}
+	resp2.Body.Close()
+	if resp2.StatusCode != 200 {
+		t.Errorf("expected 200 when ready, got %d", resp2.StatusCode)
+	}
+}