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_test.go

@@ -1,11 +1,14 @@
-package bork
+package bork_test
 
 import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+
+	"sneak.berlin/go/mfer/internal/bork"
 )
 
 func TestBuild(t *testing.T) {
-	assert.NotNil(t, ErrMissingMagic)
+	t.Parallel()
+	assert.Error(t, bork.ErrMissingMagic)
 }