Update golangci-lint to v2.12.2 with canonical config (closes #60)

Adopts golangci-lint v2.12.2 and the canonical .golangci.yml (default: all), and fixes all resulting findings across the tree.

Two intended behavior changes: absent MFFilePath.Mtime is handled explicitly in freshen, list and export rather than dereferenced (main panicked); gpg positional key IDs now follow an explicit -- end-of-options marker.

All twelve reworded user-visible error messages restored to byte-identical parity with main and pinned by tests.

internal/bork/error.go

@@ -1,15 +1,14 @@
+// Package bork defines the sentinel errors used by the manifest
+// reader and writer.
 package bork
 
 import (
 	"errors"
-	"fmt"
 )
 
 var (
-	ErrMissingMagic  = errors.New("missing magic bytes in file")
+	// ErrMissingMagic indicates the input lacks the manifest magic bytes.
+	ErrMissingMagic = errors.New("missing magic bytes in file")
+	// ErrFileTruncated indicates the input ended before the expected length.
 	ErrFileTruncated = errors.New("file/stream is truncated abnormally")
 )
-
-func Newf(format string, args ...interface{}) error {
-	return fmt.Errorf(format, args...)
-}