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/mtime.go

@@ -0,0 +1,28 @@
+package cli
+
+import (
+	"time"
+
+	"sneak.berlin/go/mfer/mfer"
+)
+
+// mtimeAbsent is printed in place of a modification time when a manifest
+// entry does not carry one.
+const mtimeAbsent = "-"
+
+// entryMtime returns the modification time recorded for a manifest entry.
+//
+// MFFilePath.Mtime is a message pointer with proto3 field presence, so an
+// absent mtime is a representable, on-the-wire-valid state. It must never
+// be conflated with a recorded mtime of the Unix epoch: callers that
+// compare mtimes have to treat "absent" as "unknown", not as
+// 1970-01-01T00:00:00Z, or every entry compares as modified. ok reports
+// whether an mtime was actually recorded.
+func entryMtime(entry *mfer.MFFilePath) (time.Time, bool) {
+	ts := entry.GetMtime()
+	if ts == nil {
+		return time.Time{}, false
+	}
+
+	return time.Unix(ts.GetSeconds(), int64(ts.GetNanos())), true
+}