feat: auto-relink skill symlinks on prefix config change (#578)

New bin/gstack-relink creates prefixed (gstack-*) or flat symlinks
based on skill_prefix config. gstack-config auto-triggers relink
when skill_prefix changes. Setup guards against recursive calls
with GSTACK_SETUP_RUNNING env var.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bin/gstack-config

@@ -41,6 +41,11 @@ case "${1:-}" in
     else
       echo "${KEY}: ${VALUE}" >> "$CONFIG_FILE"
     fi
+    # Auto-relink skills when prefix setting changes (skip during setup to avoid recursive call)
+    if [ "$KEY" = "skill_prefix" ] && [ -z "${GSTACK_SETUP_RUNNING:-}" ]; then
+      GSTACK_RELINK="$(dirname "$0")/gstack-relink"
+      [ -x "$GSTACK_RELINK" ] && "$GSTACK_RELINK" || true
+    fi
     ;;
   list)
     cat "$CONFIG_FILE" 2>/dev/null || true