chore: gitignore /.worktrees for in-repo feature worktrees

Feature work for the spec-todo roadmap is done in git worktrees under
.worktrees/ (inside the repo so the Docker dev container's /work bind mount can
build them); each feature merges back to main when it fully works.

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

Author: olwang

.gitignore

@@ -5,3 +5,6 @@ benchmarks/**/target/
 benchmarks/**/Cargo.lock
 .DS_Store
 .claude/
+
+# Feature worktrees (in-repo so the Docker bind mount can build them)
+/.worktrees/