Add tracing manifests and docs; clean up branch lint findings

Manager env block (downward-API resource attrs, commented OTLP
examples), architecture §10 + Decisions entries, README section.
Lint: goconst constants, gofmt, logcheck (Setup now takes its logger
from ctx via logf.FromContext).

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/main.go

@@ -42,6 +42,7 @@ import (
 	"sigs.k8s.io/controller-runtime/pkg/cache"
 	"sigs.k8s.io/controller-runtime/pkg/client"
 	"sigs.k8s.io/controller-runtime/pkg/healthz"
+	logf "sigs.k8s.io/controller-runtime/pkg/log"
 	"sigs.k8s.io/controller-runtime/pkg/log/zap"
 	ctrlmetrics "sigs.k8s.io/controller-runtime/pkg/metrics"
 	"sigs.k8s.io/controller-runtime/pkg/metrics/filters"
@@ -157,7 +158,7 @@ func main() {
 	// Off (no-op spans, unchanged logs) unless OTEL_* env opts in; see
 	// internal/tracing. Shutdown is called explicitly after mgr.Start
 	// returns — the os.Exit paths below skip defers.
-	tracingShutdown, err := tracing.Setup(context.Background(), setupLog,
+	tracingShutdown, err := tracing.Setup(logf.IntoContext(context.Background(), setupLog),
 		"egress-proxies-operator", version.Resolve())
 	if err != nil {
 		setupLog.Error(err, "Failed to set up tracing")