Only create log files for backup/restore/retention commands

Utility commands (schedule, snapshots, remotes, etc.) were creating
empty 0 B log files on every invocation. Now init_logging only runs
for commands that produce meaningful log output.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bin/gniza

@@ -106,7 +106,10 @@ elif [[ -f "$CONFIG_DIR/gniza.conf" ]]; then
     load_config
 fi
 
-init_logging
+# Only create log files for operations that produce meaningful output
+case "${SUBCOMMAND:-}" in
+    backup|restore|retention) init_logging ;;
+esac
 
 # ── Parse subcommand flags helper ────────────────────────────
 _parse_flag() {