chore: untrack plan.json and skills-lock.json

Both are local state files. plan.json was already gitignored,
skills-lock.json now added to .gitignore.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -1,5 +1,6 @@
-# Claude Code skills (local tooling, not project code)
+# Claude Code local files
 .agents/
+skills-lock.json
 
 # Dependencies
 node_modules/