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/snapshot/manifest.go

@@ -10,6 +10,8 @@ import (
 )
 
 // Manifest represents the structure of a snapshot's blob manifest
+//
+//nolint:tagliatelle // snake_case is the established on-disk manifest format
 type Manifest struct {
 	SnapshotID          string     `json:"snapshot_id"`
 	Timestamp           string     `json:"timestamp"`
@@ -19,6 +21,8 @@ type Manifest struct {
 }
 
 // BlobInfo represents information about a single blob in the manifest
+//
+//nolint:tagliatelle // snake_case is the established on-disk manifest format
 type BlobInfo struct {
 	Hash           string `json:"hash"`
 	CompressedSize int64  `json:"compressed_size"`
@@ -35,7 +39,9 @@ func DecodeManifest(r io.Reader) (*Manifest, error) {
 
 	// Decode JSON manifest
 	var manifest Manifest
-	if err := json.NewDecoder(zr).Decode(&manifest); err != nil {
+
+	err = json.NewDecoder(zr).Decode(&manifest)
+	if err != nil {
 		return nil, fmt.Errorf("decoding manifest: %w", err)
 	}
 
@@ -53,18 +59,21 @@ func EncodeManifest(manifest *Manifest, compressionLevel int) ([]byte, error) {
 	// Compress using zstd
 	var compressedBuf bytes.Buffer
 
-	writer, err := zstd.NewWriter(&compressedBuf, zstd.WithEncoderLevel(zstd.EncoderLevelFromZstd(compressionLevel)))
+	writer, err := zstd.NewWriter(&compressedBuf,
+		zstd.WithEncoderLevel(zstd.EncoderLevelFromZstd(compressionLevel)))
 	if err != nil {
 		return nil, fmt.Errorf("creating zstd writer: %w", err)
 	}
 
-	if _, err := writer.Write(jsonData); err != nil {
+	_, err = writer.Write(jsonData)
+	if err != nil {
 		_ = writer.Close()
 
 		return nil, fmt.Errorf("writing compressed data: %w", err)
 	}
 
-	if err := writer.Close(); err != nil {
+	err = writer.Close()
+	if err != nil {
 		return nil, fmt.Errorf("closing zstd writer: %w", err)
 	}