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/cli/cmd_targets.go

@@ -18,7 +18,7 @@ func runTargets(args []string) {
 		fmt.Fprintln(os.Stderr, "usage: logtail-cli targets [--target host:port] [--json]")
 		fs.PrintDefaults()
 	}
-	fs.Parse(args)
+	_ = fs.Parse(args) // ExitOnError: only returns nil here
 	sf.resolve(*target)
 
 	for _, addr := range sf.targets {
@@ -30,7 +30,7 @@ func runTargets(args []string) {
 		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 		resp, err := client.ListTargets(ctx, &pb.ListTargetsRequest{})
 		cancel()
-		conn.Close()
+		_ = conn.Close()
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "targets: %s: %v\n", addr, err)
 			continue
@@ -43,7 +43,7 @@ func runTargets(args []string) {
 				Addr        string `json:"addr"`
 			}
 			for _, t := range resp.Targets {
-				json.NewEncoder(os.Stdout).Encode(row{QueryTarget: addr, Name: t.Name, Addr: t.Addr})
+				_ = json.NewEncoder(os.Stdout).Encode(row{QueryTarget: addr, Name: t.Name, Addr: t.Addr})
 			}
 		} else {
 			if len(sf.targets) > 1 {