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>

internal/tracing/tracing_test.go

@@ -5,24 +5,25 @@ import (
 	"testing"
 	"time"
 
-	"github.com/go-logr/logr"
 	"go.opentelemetry.io/otel"
 	sdktrace "go.opentelemetry.io/otel/sdk/trace"
 	"go.opentelemetry.io/otel/trace/noop"
 )
 
+const testOTLPEndpoint = "http://localhost:4318"
+
 // clearOTelEnv pins every env var Setup reads, so developer shells with
 // OTEL_* set can't change test outcomes. Not parallel-safe by design
 // (t.Setenv forbids t.Parallel).
 func clearOTelEnv(t *testing.T) {
 	t.Helper()
 	for _, k := range []string{
-		"OTEL_SDK_DISABLED",
-		"OTEL_TRACES_EXPORTER",
-		"OTEL_EXPORTER_OTLP_ENDPOINT",
-		"OTEL_EXPORTER_OTLP_TRACES_ENDPOINT",
-		"OTEL_EXPORTER_OTLP_PROTOCOL",
-		"OTEL_EXPORTER_OTLP_TRACES_PROTOCOL",
+		envSDKDisabled,
+		envTracesExporter,
+		envOTLPEndpoint,
+		envOTLPTracesEndpoint,
+		envOTLPProtocol,
+		envOTLPTracesProtocol,
 	} {
 		t.Setenv(k, "")
 	}
@@ -38,50 +39,50 @@ func TestSetup_envGating(t *testing.T) {
 		{name: "no env means disabled", env: nil, wantEnabled: false},
 		{
 			name:        "endpoint enables otlp",
-			env:         map[string]string{"OTEL_EXPORTER_OTLP_ENDPOINT": "http://localhost:4318"},
+			env:         map[string]string{envOTLPEndpoint: testOTLPEndpoint},
 			wantEnabled: true,
 		},
 		{
 			name:        "traces endpoint enables otlp",
-			env:         map[string]string{"OTEL_EXPORTER_OTLP_TRACES_ENDPOINT": "http://localhost:4318"},
+			env:         map[string]string{envOTLPTracesEndpoint: testOTLPEndpoint},
 			wantEnabled: true,
 		},
 		{
 			name:        "explicit console exporter",
-			env:         map[string]string{"OTEL_TRACES_EXPORTER": "console"},
+			env:         map[string]string{envTracesExporter: exporterConsole},
 			wantEnabled: true,
 		},
 		{
 			name:        "grpc protocol",
-			env:         map[string]string{"OTEL_TRACES_EXPORTER": "otlp", "OTEL_EXPORTER_OTLP_PROTOCOL": "grpc"},
+			env:         map[string]string{envTracesExporter: exporterOTLP, envOTLPProtocol: "grpc"},
 			wantEnabled: true,
 		},
 		{
 			name: "exporter none wins over endpoint",
 			env: map[string]string{
-				"OTEL_TRACES_EXPORTER":        "none",
-				"OTEL_EXPORTER_OTLP_ENDPOINT": "http://localhost:4318",
+				envTracesExporter: exporterNone,
+				envOTLPEndpoint:   testOTLPEndpoint,
 			},
 			wantEnabled: false,
 		},
 		{
 			name: "OTEL_SDK_DISABLED wins over everything",
 			env: map[string]string{
-				"OTEL_SDK_DISABLED":           "true",
-				"OTEL_EXPORTER_OTLP_ENDPOINT": "http://localhost:4318",
+				envSDKDisabled:  "true",
+				envOTLPEndpoint: testOTLPEndpoint,
 			},
 			wantEnabled: false,
 		},
 		{
 			name:    "unsupported exporter errors",
-			env:     map[string]string{"OTEL_TRACES_EXPORTER": "jaeger"},
+			env:     map[string]string{envTracesExporter: "jaeger"},
 			wantErr: true,
 		},
 		{
 			name: "unsupported protocol errors",
 			env: map[string]string{
-				"OTEL_TRACES_EXPORTER":         "otlp",
-				"OTEL_EXPORTER_OTLP_PROTOCOL": "http/json",
+				envTracesExporter: exporterOTLP,
+				envOTLPProtocol:   "http/json",
 			},
 			wantErr: true,
 		},
@@ -100,7 +101,7 @@ func TestSetup_envGating(t *testing.T) {
 			before := noop.NewTracerProvider()
 			otel.SetTracerProvider(before)
 
-			shutdown, err := Setup(context.Background(), logr.Discard(), "test-svc", "abc123")
+			shutdown, err := Setup(context.Background(), "test-svc", "abc123")
 			if tc.wantErr {
 				if err == nil {
 					t.Fatal("want error, got nil")