RELEASE 1.0.1: v2 log format, source_tag-labeled metrics, lint cleanup

Wire-format and metric overhaul. Both file and UDP ingest now share one
versioned ParseLine that dispatches on the v<N>\t prefix; v1 stays
unchanged, v2 adds $bytes_sent (replacing $body_bytes_sent),
$request_length, $upstream_response_time, and $upstream_status. File
ingest gains the same versioning, and the legacy positional file format
is removed (no live deployments).

Prometheus exposition is rewritten:

  - nginx_http_bytes_sent and nginx_http_request_duration_seconds gain
    a source_tag label.
  - nginx_http_requests_by_source_total gains status_class.
  - New v2-only metrics: nginx_http_request_bytes,
    nginx_http_upstream_duration_seconds,
    nginx_http_upstream_requests_total{status_class}.
  - Dropped nginx_http_response_body_bytes_by_source (subsumed by the
    dual-labeled bytes_sent metric).

Adds 'make fixstyle' (gofmt -w) and clears all golangci-lint findings
across the repo (errcheck, S1001, ST1005, unused).

Docs in design.md FR-2/FR-8 and user-guide.md are rewritten to present
v2 as the recommended log format.

cmd/aggregator/aggregator_test.go

@@ -262,7 +262,7 @@ func startFakeCollector(t *testing.T, snaps []*pb.Snapshot) string {
 	}
 	srv := grpc.NewServer()
 	pb.RegisterLogtailServiceServer(srv, &fakeCollector{snaps: snaps})
-	go srv.Serve(lis)
+	go func() { _ = srv.Serve(lis) }()
 	t.Cleanup(srv.Stop)
 	return lis.Addr().String()
 }
@@ -310,7 +310,7 @@ func TestGRPCEndToEnd(t *testing.T) {
 	}
 	grpcSrv := grpc.NewServer()
 	pb.RegisterLogtailServiceServer(grpcSrv, NewServer(cache, "agg-test", NewTargetRegistry(nil)))
-	go grpcSrv.Serve(lis)
+	go func() { _ = grpcSrv.Serve(lis) }()
 	defer grpcSrv.Stop()
 
 	conn, err := grpc.NewClient(lis.Addr().String(),
@@ -318,7 +318,7 @@ func TestGRPCEndToEnd(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 	client := pb.NewLogtailServiceClient(conn)
 	qctx, qcancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer qcancel()