feat: robust username detection and CI updates

Add platform-specific username detection for Control D metadata:
- macOS: directory services (dscl) with console user fallback
- Linux: systemd loginctl, utmp, /etc/passwd traversal
- Windows: WTS session enumeration, registry, token lookup

discover_user_others.go

@@ -0,0 +1,13 @@
+//go:build !windows && !linux && !darwin
+
+package ctrld
+
+import "context"
+
+// DiscoverMainUser returns "unknown" for unsupported platforms.
+// This is a stub implementation for platforms where username detection
+// is not yet implemented.
+func DiscoverMainUser(ctx context.Context) string {
+	LoggerFromCtx(ctx).Debug().Msg("username discovery not implemented for this platform")
+	return "unknown"
+}