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/list.go

@@ -25,6 +25,7 @@ func (mfa *CLIApp) listManifestOperation(ctx *cli.Context) error {
 	if err != nil {
 		return fmt.Errorf("list: %w", err)
 	}
+
 	defer func() { _ = rc.Close() }()
 
 	manifest, err := mfer.NewManifestFromReader(rc)
@@ -42,10 +43,17 @@ func (mfa *CLIApp) listManifestOperation(ctx *cli.Context) error {
 
 	for _, f := range files {
 		if longFormat {
-			mtime := time.Unix(f.Mtime.Seconds, int64(f.Mtime.Nanos))
-			_, _ = fmt.Fprintf(mfa.Stdout, "%d\t%s\t%s%s", f.Size, mtime.Format(time.RFC3339), f.Path, lineEnd)
+			// An entry may legitimately carry no mtime; render that as
+			// mtimeAbsent rather than as the Unix epoch.
+			mtimeStr := mtimeAbsent
+			if mtime, ok := entryMtime(f); ok {
+				mtimeStr = mtime.Format(time.RFC3339)
+			}
+
+			_, _ = fmt.Fprintf(mfa.Stdout, "%d\t%s\t%s%s",
+				f.GetSize(), mtimeStr, f.GetPath(), lineEnd)
 		} else {
-			_, _ = fmt.Fprintf(mfa.Stdout, "%s%s", f.Path, lineEnd)
+			_, _ = fmt.Fprintf(mfa.Stdout, "%s%s", f.GetPath(), lineEnd)
 		}
 	}