fix: improve feature completeness and correctness

Pass 1 (Feature & Completeness):
- Replace apt CLI with apt-get for listUpgrades (apt CLI is unstable for scripting)
- Verify snapd daemon is running in snap Available() check
- Add ErrDaemonNotRunning sentinel error for daemon-dependent managers
- Fix staticcheck S1011: replace loop with append(keys, matches...)
- Fix staticcheck SA1012: use context.TODO() instead of nil in dpkg tests

snap/snap_linux.go

@@ -13,8 +13,12 @@ import (
 )
 
 func available() bool {
-	_, err := exec.LookPath("snap")
-	return err == nil
+	if _, err := exec.LookPath("snap"); err != nil {
+		return false
+	}
+	// Verify snapd is running by checking snap version (requires daemon).
+	cmd := exec.Command("snap", "version")
+	return cmd.Run() == nil
 }
 
 func run(ctx context.Context, args []string) (string, error) {