gitignore: exclude local-only skills from skills-win sync

BMSv4_pincfg and cubemx-min-prj were removed upstream but are still
used on this machine; keep them on disk and untracked. Also ignore
.stfolder (syncthing metadata) and .claude.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -355,3 +355,9 @@ web-design-guidelines/SKILL.md
 .system/openai-docs/
 .system/review-agent/
 __pycache__/
+
+# --- local-only skills (not synced to skills-win) ---
+/BMSv4_pincfg/
+/cubemx-min-prj/
+/.stfolder/
+/.claude/