chore: untrack .claude/ session artifact, add to .gitignore

`git add -A` in the previous commit accidentally swept in
.claude/scheduled_tasks.lock — a Claude Code local runtime artifact. Remove
it from the index and gitignore the .claude/ dir so it can't recur.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -14,3 +14,6 @@
 
 # Vim help tags (regenerated by :helptags)
 doc/tags
+
+# Claude Code local session artifacts
+.claude/