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-relink

@@ -66,6 +66,9 @@ for skill_dir in "$INSTALL_DIR"/*/; do
   SKILL_COUNT=$((SKILL_COUNT + 1))
 done
 
+# Patch SKILL.md name: fields to match prefix setting
+"$INSTALL_DIR/bin/gstack-patch-names" "$INSTALL_DIR" "$PREFIX"
+
 if [ "$PREFIX" = "true" ]; then
   echo "Relinked $SKILL_COUNT skills as gstack-*"
 else