feat: add centralized logging handler with structured log format

- Add log() function with info|warn|error|debug levels
- Log format: timestamp level component message
- Sensitive values (tokens, passwords, secrets) are masked automatically
- Fix mask_sensitive_vars to handle empty input gracefully

Implements: shoko/kugetsu#118

skills/kugetsu/scripts/kugetsu-config.sh

@@ -32,7 +32,7 @@ if [ -f "$KUGETSU_DIR/config" ]; then
 fi
 
 mask_sensitive_vars() {
-    local line="$1"
+    local line="${1:-}"
     for var in GITEA_TOKEN GITHUB_TOKEN GITLAB_TOKEN API_KEY PASSWORD TOKEN SECRET; do
         if [[ "$line" =~ $var ]]; then
             line=$(echo "$line" | sed -E "s/=.*/=***MASKED***/")

skills/kugetsu/scripts/kugetsu-log.sh

@@ -1,6 +1,40 @@
 #!/bin/bash
 set -euo pipefail
 
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+source "$SCRIPT_DIR/kugetsu-config.sh"
+
+log() {
+    local level="${1:-}"
+    local component="${2:-}"
+    local message="${3:-}"
+    local timestamp
+    timestamp=$(date -Iseconds)
+
+    case "$level" in
+        info|warn|error|debug) ;;
+        *)
+            echo "Error: log level must be info|warn|error|debug" >&2
+            return 1
+            ;;
+    esac
+
+    if [ -z "$message" ]; then
+        message="$component"
+        component="${level}"
+        level="info"
+    fi
+
+    local masked
+    masked=$(mask_sensitive_vars "$message")
+    echo "[$timestamp] $level $component $masked"
+}
+
+log_debug() { log "debug" "$1" "${2:-}"; }
+log_info()  { log "info"  "$1" "${2:-}"; }
+log_warn()  { log "warn"  "$1" "${2:-}"; }
+log_error() { log "error" "$1" "${2:-}"; }
+
 cmd_logs() {
     local count="${1:-10}"