fix: detect .worktrees/ ignore via git check-ignore

[CoodingPenguin]

Summary

• Replace the .gitignore-grep guard for the default .worktrees/ root with a git check-ignore query, so global excludes and .git/info/exclude count as valid
• Refine the rejection message to make the broader scope explicit
• Add a bats case covering the .git/info/exclude path that was previously rejected
• Update both READMEs to describe the new acceptance rules

Changes

• cwt.sh: _cwt_ensure_default_worktree_ignored now delegates to git -C "$git_root" check-ignore -q .worktrees/, dropping the literal .gitignore regex scan
• Messaging: warn/error strings switched from "in .gitignore" / "Refusing to edit .gitignore automatically" to "to be gitignored (local, global, or info/exclude)" / "Add .worktrees/ to your gitignore and rerun"
• tests/cwt_new.bats: update the existing rejection assertions to the new strings; add accepts .worktrees/ ignored via .git/info/exclude covering the new code path
• README.md / README.ko.md: clarify that any gitignore source (repo .gitignore, .git/info/exclude, global excludes) satisfies the requirement

Test plan

• npx bats tests/cwt_new.bats — 44/44 passing, including the new .git/info/exclude case
• Manual: empty repo .gitignore + global core.excludesFile listing .worktrees/ → cwt new proceeds
• Manual: .git/info/exclude only → cwt new proceeds
• Manual: no source ignores .worktrees/ → blocked with the new message