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

errors.go

@@ -28,4 +28,8 @@ var (
 	// ErrManagerNotFound is returned by detect when no supported package
 	// manager can be found on the system.
 	ErrManagerNotFound = errors.New("no supported package manager found")
+
+	// ErrDaemonNotRunning is returned when a package manager's required
+	// daemon (e.g. snapd) is not running.
+	ErrDaemonNotRunning = errors.New("package manager daemon is not running")
 )