Worktree path checks now handle case differences and equivalent paths

A worktree is an extra working copy of a git repository in its own folder. Several checks compare folder paths: entering a worktree, the managed .claude/worktrees folder, and the link from a worktree back to its repository. These checks used to compare paths as plain text. They now ignore upper and lower case on Windows and macOS, or on any platform where a check asks for it. They then confirm a match by checking that both paths point to the same item on disk. Looking up a worktree's lock also falls back to a case-insensitive match.