Fix all linter errors

- Add explicit error ignoring with _ = for Close/Remove calls
- Rename WriteTo to Write to avoid io.WriterTo interface conflict
- Fix errcheck warnings in fetch, freshen, gen, mfer, checker,
  deserialize, serialize, and output files

internal/cli/fetch.go

@@ -48,7 +48,7 @@ func (mfa *CLIApp) fetchManifestOperation(ctx *cli.Context) error {
 	if err != nil {
 		return fmt.Errorf("failed to fetch manifest: %w", err)
 	}
-	defer resp.Body.Close()
+	defer func() { _ = resp.Body.Close() }()
 
 	if resp.StatusCode != http.StatusOK {
 		return fmt.Errorf("failed to fetch manifest: HTTP %d", resp.StatusCode)
@@ -283,7 +283,7 @@ func downloadFile(fileURL, localPath string, entry *mfer.MFFilePath, progress ch
 	if err != nil {
 		return err
 	}
-	defer resp.Body.Close()
+	defer func() { _ = resp.Body.Close() }()
 
 	if resp.StatusCode != http.StatusOK {
 		return fmt.Errorf("HTTP %d", resp.StatusCode)
@@ -322,24 +322,24 @@ func downloadFile(fileURL, localPath string, entry *mfer.MFFilePath, progress ch
 
 	// If copy failed, clean up temp file and return error
 	if copyErr != nil {
-		os.Remove(tmpPath)
+		_ = os.Remove(tmpPath)
 		return copyErr
 	}
 	if closeErr != nil {
-		os.Remove(tmpPath)
+		_ = os.Remove(tmpPath)
 		return closeErr
 	}
 
 	// Verify size
 	if written != expectedSize {
-		os.Remove(tmpPath)
+		_ = os.Remove(tmpPath)
 		return fmt.Errorf("size mismatch: expected %d bytes, got %d", expectedSize, written)
 	}
 
 	// Encode computed hash as multihash
 	computed, err := multihash.Encode(h.Sum(nil), multihash.SHA2_256)
 	if err != nil {
-		os.Remove(tmpPath)
+		_ = os.Remove(tmpPath)
 		return fmt.Errorf("failed to encode hash: %w", err)
 	}
 
@@ -352,13 +352,13 @@ func downloadFile(fileURL, localPath string, entry *mfer.MFFilePath, progress ch
 		}
 	}
 	if !hashMatch {
-		os.Remove(tmpPath)
+		_ = os.Remove(tmpPath)
 		return fmt.Errorf("hash mismatch")
 	}
 
 	// Rename temp file to final path
 	if err := os.Rename(tmpPath, localPath); err != nil {
-		os.Remove(tmpPath)
+		_ = os.Remove(tmpPath)
 		return fmt.Errorf("failed to rename temp file: %w", err)
 	}