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/provider/metrics.go

@@ -50,17 +50,25 @@ func (i *instrumented) record(op string, err error) {
 	i.rec.ProviderRequest(i.name, op, resultLabel(err))
 }
 
+const (
+	resultOK            = "ok"
+	resultNotFound      = "not_found"
+	resultQuotaExceeded = "quota_exceeded"
+	resultPermanent     = "permanent"
+	resultTransient     = "transient"
+)
+
 func resultLabel(err error) string {
 	switch Class(err) {
 	case nil:
-		return "ok"
+		return resultOK
 	case ErrNotFound:
-		return "not_found"
+		return resultNotFound
 	case ErrQuotaExceeded:
-		return "quota_exceeded"
+		return resultQuotaExceeded
 	case ErrPermanent:
-		return "permanent"
+		return resultPermanent
 	default:
-		return "transient"
+		return resultTransient
 	}
 }