feat(skills): add list_existing_tags.sh script for future tag normalization

.claude/skills/mt-add-tags/scripts/list_existing_tags.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+# List existing tags in the repo ranked by frequency.
+# Usage: list_existing_tags.sh
+# Outputs: tag count and name, sorted most-used first (top 40)
+#
+# NOTE: This script is NOT currently used by the mt-add-tags skill.
+# Tag normalization is disabled until existing posts have standardized tags.
+# To enable, uncomment step 4a in SKILL.md.
+
+set -euo pipefail
+
+PROJECT_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/../../../.." && pwd)"
+CONTENT_DIR="$PROJECT_ROOT/content/post"
+
+grep -rh "^tags:" "$CONTENT_DIR" \
+  | tr ',' '\n' \
+  | grep -oP '(?<=")[^"]+(?=")' \
+  | sort \
+  | uniq -c \
+  | sort -rn \
+  | head -40