always add worktree ignore rule to .gitignore

Drop the interactive .gitignore/.git-info-exclude choice: the ignore rule for .entire/worktrees/ is now always appended to the repo-root .gitignore when not already ignored. Persisting the rule is the user's choice, made by committing or discarding the .gitignore change. This removes the prompt, the exclude writer, and --force's second meaning.