fix: patch name: in SKILL.md frontmatter for prefix mode (#620, #578)

./setup --prefix creates gstack-* symlinks but SKILL.md still says
name: qa, so Claude Code ignores the prefix. Now:
- New bin/gstack-patch-names shared helper patches name: field via sed
- setup calls it after link_claude_skill_dirs
- gstack-relink calls it after symlink loop
- gen-skill-docs.ts prints warning when skill_prefix is true

Edge cases: gstack-upgrade not double-prefixed, root gstack skill
never prefixed, prefix removal restores original names, SKILL.md
without frontmatter is a safe no-op.

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

bin/gstack-patch-names

@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+# gstack-patch-names — patch name: field in SKILL.md frontmatter for prefix mode
+# Usage: gstack-patch-names <gstack-dir> <true|false|1|0>
+set -euo pipefail
+
+GSTACK_DIR="$1"
+DO_PREFIX="$2"
+
+# Normalize prefix arg
+case "$DO_PREFIX" in true|1) DO_PREFIX=1 ;; *) DO_PREFIX=0 ;; esac
+
+PATCHED=0
+for skill_dir in "$GSTACK_DIR"/*/; do
+  [ -f "$skill_dir/SKILL.md" ] || continue
+  dir_name="$(basename "$skill_dir")"
+  [ "$dir_name" = "node_modules" ] && continue
+  cur=$(grep -m1 '^name:' "$skill_dir/SKILL.md" 2>/dev/null | sed 's/^name:[[:space:]]*//' | tr -d '[:space:]' || true)
+  [ -z "$cur" ] && continue
+  [ "$cur" = "gstack" ] && continue  # never prefix root skill
+  if [ "$DO_PREFIX" -eq 1 ]; then
+    case "$cur" in gstack-*) continue ;; esac
+    new="gstack-$cur"
+  else
+    case "$cur" in gstack-*) ;; *) continue ;; esac
+    [ "$dir_name" = "$cur" ] && continue  # inherently prefixed (gstack-upgrade)
+    new="${cur#gstack-}"
+  fi
+  tmp="$(mktemp "${skill_dir}/SKILL.md.XXXXXX")"
+  sed "1,/^---$/s/^name:[[:space:]]*${cur}/name: ${new}/" "$skill_dir/SKILL.md" > "$tmp" && mv "$tmp" "$skill_dir/SKILL.md"
+  PATCHED=$((PATCHED + 1))
+done
+if [ "$PATCHED" -gt 0 ]; then
+  echo "  patched name: field in $PATCHED skills"
+fi