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.

mfer/url_test.go

@@ -1,3 +1,4 @@
+//nolint:testpackage // white-box tests exercise unexported internals
 package mfer
 
 import (
@@ -8,19 +9,27 @@ import (
 )
 
 func TestBaseURLJoinPath(t *testing.T) {
+	t.Parallel()
+
 	tests := []struct {
 		base     BaseURL
 		path     RelFilePath
 		expected string
 	}{
-		{"https://example.com/dir/", "file.txt", "https://example.com/dir/file.txt"},
-		{"https://example.com/dir", "file.txt", "https://example.com/dir/file.txt"},
+		{"https://example.com/dir/", testFileName, "https://example.com/dir/file.txt"},
+		{"https://example.com/dir", testFileName, "https://example.com/dir/file.txt"},
 		{"https://example.com/", "sub/file.txt", "https://example.com/sub/file.txt"},
-		{"https://example.com/dir/", "file with spaces.txt", "https://example.com/dir/file%20with%20spaces.txt"},
+		{
+			"https://example.com/dir/",
+			"file with spaces.txt",
+			"https://example.com/dir/file%20with%20spaces.txt",
+		},
 	}
 
 	for _, tt := range tests {
 		t.Run(string(tt.base)+"+"+string(tt.path), func(t *testing.T) {
+			t.Parallel()
+
 			result, err := tt.base.JoinPath(tt.path)
 			require.NoError(t, err)
 			assert.Equal(t, tt.expected, string(result))
@@ -29,16 +38,22 @@ func TestBaseURLJoinPath(t *testing.T) {
 }
 
 func TestBaseURLString(t *testing.T) {
+	t.Parallel()
+
 	b := BaseURL("https://example.com/")
 	assert.Equal(t, "https://example.com/", b.String())
 }
 
 func TestFileURLString(t *testing.T) {
+	t.Parallel()
+
 	f := FileURL("https://example.com/file.txt")
 	assert.Equal(t, "https://example.com/file.txt", f.String())
 }
 
 func TestManifestURLString(t *testing.T) {
+	t.Parallel()
+
 	m := ManifestURL("https://example.com/index.mf")
 	assert.Equal(t, "https://example.com/index.mf", m.String())
 }