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>

setup

@@ -567,6 +567,7 @@ if [ "$INSTALL_CLAUDE" -eq 1 ]; then
       cleanup_prefixed_claude_symlinks "$SOURCE_GSTACK_DIR" "$INSTALL_SKILLS_DIR"
     fi
     link_claude_skill_dirs "$SOURCE_GSTACK_DIR" "$INSTALL_SKILLS_DIR"
+    "$SOURCE_GSTACK_DIR/bin/gstack-patch-names" "$SOURCE_GSTACK_DIR" "$SKILL_PREFIX"
     if [ "$LOCAL_INSTALL" -eq 1 ]; then
       echo "gstack ready (project-local)."
       echo "  skills: $INSTALL_SKILLS_DIR"