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/cli/entry.go

@@ -7,15 +7,18 @@ import (
 	"github.com/spf13/afero"
 )
 
-// NO_COLOR disables colored output when set. Automatically true if the
+// NoColor disables colored output when set. Automatically true if the
 // NO_COLOR environment variable is present (per https://no-color.org/).
-var NO_COLOR bool
+//
+//nolint:gochecknoglobals // process-wide setting derived from the environment
+var NoColor = noColorEnvSet()
 
-func init() {
-	NO_COLOR = false
-	if _, exists := os.LookupEnv("NO_COLOR"); exists {
-		NO_COLOR = true
-	}
+// noColorEnvSet reports whether the NO_COLOR environment variable is
+// present.
+func noColorEnvSet() bool {
+	_, exists := os.LookupEnv("NO_COLOR")
+
+	return exists
 }
 
 // RunOptions contains all configuration for running the CLI application.
@@ -64,5 +67,6 @@ func RunWithOptions(opts *RunOptions) int {
 	}
 
 	m.run(opts.Args)
+
 	return m.exitCode
 }