Exit non-zero when snapshot create --skip-errors skips anything it cannot read (closes #310)

A backup under --skip-errors printed its skip errors and failed count,
then "Finished successfully.", and exited 0, so a cron wrapper could
not tell it from a complete backup. A directory the walk could not list
was not counted at all.

Each entry the walk skips is now counted as examined and failed. When
--skip-errors skipped anything it could not read or list, the run keeps
its snapshots, runs the post-backup prune as before, and then returns
an error the command reports as "Backup completed, but N file(s) could
not be read". A file removed during the run is still counted as failed
in the summary but leaves the exit status at 0, with or without the
flag.

Model: opus-5-5

internal/cli/snapshot.go

@@ -115,6 +115,13 @@ on Linux), then /etc/vaultik/config.yml.`,
 					return
 				}
 
+				if errors.Is(err, vaultik.ErrFilesUnreadable) {
+					log.Error("Backup completed with unreadable files", "error", err)
+					ReportErrorf("Backup completed, but %v", err)
+
+					return
+				}
+
 				log.Error("Snapshot creation failed", "error", err)
 				ReportErrorf("Snapshot creation failed: %v", err)
 			})