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/frontend/frontend_test.go

@@ -52,7 +52,7 @@ func startFake(t *testing.T, fs *fakeServer) string {
 	}
 	srv := grpc.NewServer()
 	pb.RegisterLogtailServiceServer(srv, fs)
-	go srv.Serve(lis)
+	go func() { _ = srv.Serve(lis) }()
 	t.Cleanup(srv.GracefulStop)
 	return lis.Addr().String()
 }
@@ -541,7 +541,7 @@ func TestDialFake(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 
 	_ = client
 	// If we got here without error, the fake server is reachable.