chore: remove worktree tracking from git, add to .gitignore

- Remove .claude/worktrees/benchmark and .claude/worktrees/npalign from tracking
- These are local worktree references, not meant to be shared
- Added .claude/worktrees/ to .gitignore to prevent future commits

Author: Nucs

.claude/worktrees/benchmark

@@ -357,3 +357,6 @@ test/NumSharp.UnitTest/DevTests*.cs
 
 # Local design documents (not tracked in git)
 docs/DESIGN.md
+
+# Claude Code worktrees (local only)
+.claude/worktrees/