Remediate all lint findings under the canonical golangci-lint config

Fix every finding surfaced by the canonical .golangci.yml with
golangci-lint v2.12.2 (refs #61), behavior-preserving throughout:

- err113: dynamic errors replaced with package-level sentinels and %w
  wrapping; direct comparisons converted to errors.Is
- goprintffuncname: printf-style helpers renamed with an f suffix
  (ui.Writer message methods, cli.ReportErrorf, database.Fatalf,
  vaultik stdoutf) and all call sites updated
- revive: stuttering type names renamed (blob.Handler, blob.WithReader,
  blob.ChunkPosition, storage.URL, storage.Info), doc comments added,
  unused parameters blanked, package comments added
- contextcheck/noctx: ctx threaded through blob.Packer
  (AddChunk/Flush/FinalizeBlob/PackChunks) and scanner call sites;
  context-aware exec and sql variants used
- funlen/cyclop/gocognit/nestif/dupl: oversized or duplicated
  functions split into focused helpers across production and test code
- paralleltest/tparallel/thelper/usetesting/testpackage: tests
  parallelized where safe (global log.Initialize kept in the serial
  phase), helpers marked, t.TempDir adopted, external test packages
  where only exported API is used
- gosec: integer conversions clamped or justified, header timeouts
  added, remaining findings suppressed with per-site justifications
- mnd/goconst/lll/wsl_v5/nlreturn/noinlineerr/errcheck and other
  mechanical findings fixed directly

Remove the deprecated log.LogOptions alias (callers migrated to
log.Options). make check is green.

internal/s3/client_test.go

@@ -9,6 +9,7 @@ import (
 	"sneak.berlin/go/vaultik/internal/s3"
 )
 
+//nolint:paralleltest // test servers share a fixed localhost port
 func TestClient(t *testing.T) {
 	ts := NewTestServer(t)
 	defer func() {
@@ -33,11 +34,21 @@ func TestClient(t *testing.T) {
 		t.Fatalf("failed to create client: %v", err)
 	}
 
-	// Test PutObject
 	testKey := "foo/bar.txt"
 	testData := []byte("test data")
 
-	err = client.PutObject(ctx, testKey, bytes.NewReader(testData))
+	verifyPutGetHead(ctx, t, client, testKey, testData)
+	verifyListAndDelete(ctx, t, client, testKey)
+}
+
+// verifyPutGetHead uploads an object, reads it back, and checks existence.
+func verifyPutGetHead(
+	ctx context.Context, t *testing.T, client *s3.Client,
+	testKey string, testData []byte,
+) {
+	t.Helper()
+
+	err := client.PutObject(ctx, testKey, bytes.NewReader(testData))
 	if err != nil {
 		t.Fatalf("failed to put object: %v", err)
 	}
@@ -72,8 +83,15 @@ func TestClient(t *testing.T) {
 	if !exists {
 		t.Error("expected object to exist")
 	}
+}
+
+// verifyListAndDelete lists the object's prefix, deletes it, and checks
+// it is gone.
+func verifyListAndDelete(
+	ctx context.Context, t *testing.T, client *s3.Client, testKey string,
+) {
+	t.Helper()
 
-	// Test ListObjects
 	keys, err := client.ListObjects(ctx, "foo/")
 	if err != nil {
 		t.Fatalf("failed to list objects: %v", err)
@@ -94,7 +112,7 @@ func TestClient(t *testing.T) {
 	}
 
 	// Verify deletion
-	exists, err = client.HeadObject(ctx, testKey)
+	exists, err := client.HeadObject(ctx, testKey)
 	if err != nil {
 		t.Fatalf("failed to head object after deletion: %v", err)
 	}