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.go

@@ -32,12 +32,14 @@ func (b BaseURL) JoinPath(path RelFilePath) (FileURL, error) {
 	for i, seg := range segments {
 		segments[i] = url.PathEscape(seg)
 	}
+
 	ref, err := url.Parse(strings.Join(segments, "/"))
 	if err != nil {
 		return "", err
 	}
 
 	resolved := base.ResolveReference(ref)
+
 	return FileURL(resolved.String()), nil
 }