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.

config/config_test.go

@@ -0,0 +1,42 @@
+package config
+
+import (
+	"os"
+	"testing"
+	"time"
+)
+
+func TestLoadDefaults(t *testing.T) {
+	s := Load()
+	if s.ServiceName != "handler" {
+		t.Errorf("expected default ServiceName 'handler', got %q", s.ServiceName)
+	}
+	if s.HealthPort != 8080 {
+		t.Errorf("expected default HealthPort 8080, got %d", s.HealthPort)
+	}
+	if s.HTTPTimeout != 60*time.Second {
+		t.Errorf("expected default HTTPTimeout 60s, got %v", s.HTTPTimeout)
+	}
+}
+
+func TestLoadFromEnv(t *testing.T) {
+	os.Setenv("SERVICE_NAME", "test-svc")
+	os.Setenv("HEALTH_PORT", "9090")
+	os.Setenv("OTEL_ENABLED", "false")
+	defer func() {
+		os.Unsetenv("SERVICE_NAME")
+		os.Unsetenv("HEALTH_PORT")
+		os.Unsetenv("OTEL_ENABLED")
+	}()
+
+	s := Load()
+	if s.ServiceName != "test-svc" {
+		t.Errorf("expected ServiceName 'test-svc', got %q", s.ServiceName)
+	}
+	if s.HealthPort != 9090 {
+		t.Errorf("expected HealthPort 9090, got %d", s.HealthPort)
+	}
+	if s.OTELEnabled {
+		t.Error("expected OTELEnabled false")
+	}
+}